L-iaCKAlIICAL    PROPZRTISS    OF   *,,OOD 


By:   A.  L.  Heim,  A.  C.  Znauss,  and 
Louis  Seutter. 


Report  #145 


Forest  Products  Lab. 


Agric. -Forestry.  Main  Librar> 


REPORT  No.  145 


INTERNAL  STRESSES  IN  LAMINATED  CONSTRUCTION 


By  A.  L.  HEIM,  A.  C.  KNAUSS,  and  LOUIS  SEUTTER 
Forest  Products  Laboratory 


86540—22 1 


. 

E  OP M.  AGRICULTURE 
OF  CALIFORNIA 


Agric.- Forestry.  Main 


V 


REPORT  No.  145. 


INTERNAL  STRESSES  IN  LAMINATED  CONSTRUCTION. 

By  Forest  Products  Laboratory. 


INTRODUCTION. 

This  report,  submitted  to  the  National  Advisory  Committee  for  Aeronautics  for  publica- 
tion, covers  work  conducted  by  the  Forest  Products  Laboratory  of  the  United  States  Forest 
Service  at  the  request  of  and  with  funds  provided  by  the  Bureau  of  Engineering  of  the  Navy 

Department. 

SUMMARY. 

The  report  reviews  the  procedure  employed  in  an  investigation  of  the  sources  and  influence 
of  internal  stresses  in  laminated  construction,  and  discusses  the  influence  of  shrinkage  and 
swelling  stresses  caused  by  atmospheric  conditions  upon  the  tensile  strength  across  grain  in 
laminated  construction  with  special  reference  to  airplane  propellers. 

The  investigation  covered  three  sources  of  internal  stress,  namely,  the  combination  of 
plain-sawed  and  quarter-sawed  material  in  the  same  construction,  the  gluing  together  of  lami- 
nations of  different  moisture  contents,  and  the  gluing  together  of  laminations  of  different  densi- 
ties. The  following  species  were  studied : 

Central  American  mahogany  (Swietenia  maJiogani) . 
African  mahogany  (Khaya  senegalensis) . 
Philippine  mahogany  (SJiorea  sp.). 
Yellow  poplar  (Liriodendron  tulipifera) . 
.     Hard  maple  ( Acer  saccharum) . 
Yellow  birch  (Betula  sp.) . 
Red  gum  (Liquidambar  styraciflua) . 
Northern  white  oak  (Quercus  sp.). 
Northern  red  oak  (Quercus  sp.). 

Glued  specimens  and  free  specimens,  made  up  under  various  manufacturing  conditions, 
were  subjected  to  various  climatic  changes  inducing  internal  stresses  and  then  were  tested. 
The  strength  of  free  unstressed  pieces  served  as  a  standard  of  comparison  for  glued  pieces  and 
indicated  what  internal  stresses  were  developed  in  the  glued  construction. 

The  following  recommendations  as  to  propeller  specifications  are  made  for  the  species 
studied: 

1.  That  all  propellers  be  covered  with  aluminum  leaf  coating  or  other  approved  finish 
which  will  prevent,  so  far  as  possible,  any  gain  or  loss  in  moisture  content  of  the  propeller. 

2.  That  for  the  most  extreme  conditions  of  service  propellers  be  made  entirely  of  quarter- 
sawed  material. 

3.  That  for  moderate  conditions  of  service  propellers  made  entirely  from  plain-sawed  stock 
be  permitted;  -provided  they  are  well  protected  against  moisture  change. 

4.  That  for  species  in  which  the  ratio  of  radial  to  tangential  shrinkage  exceeds  0.75  the 
mixing  of  plain-sawed  and  quarter-sawed  stock  be  permitted  in  propellers  for  moderate  service, 
provided  that  they  are  well  protected  against  moisture  change. 

5.  That  all  propeller  stock  be  allowed  to  come  to  equilibrium  under  fixed  conditions  of 
temperature  and  relative  humidity  before  gluing. 

6.  That  density  specifications  be  such  as  to  eliminate  all  brash  material,  but  not  to  require 
matching  for  density. 

7.  That  moisture  content  of  wood,  gluing  conditions,  and  protective  coating  be  such  that 
the  mositure  content  of  the  propellers  will  not  exceed  15  per  cent  at  any  time.     Beyond  this 
point  animal  glue  is  not  likely  to  give  satisfactory  results. 


510095 


-          NATIONAL  ADVISORY  COMMITTEE  FOR  AERONAUTICS. 


These  recommendations  are  based  on  the  following  conclusions,  which  appear  to  be  war- 
ranted from  a  careful  analysis  of  the  data  obtained  in  this  study: 

1.  Tensile  strength  across  grain  (across  the  face  of  the  board)  for  quarter-sawed  lumber 
is  greater  than  for  plain-sawed  lumber.     Plain-sawed  lumber  may  be  from  20  to  50  per  cent 
weaker  across  the  grain,  depending  upon  the  species  and  method  of  drying. 

2.  The  gluing  together  of  plain-sawed  and  quarter-sawed  stock  gives  rise  to  internal  stresses 
through  the  unequal  swelling  and  shrinking  which  takes  place  with  changes  in  moisture  content 
and  results  in  a  weakening  across  grain  of  the  laminated  structure. 

3.  The  gluing  together  of  laminations  of  different  moisture  contents  gives  rise  to  internal 
stresses  on  account  of  the  unequal  swelling  or  shrinking  which  takes  place  as  all  the  lamina- 
tions approach  a  common  moisture  content,  and  results  in  a  weakening  across  grain  of  the 
structure,  which  may  be  of  sufficient  magnitude  to  cause  rupture  of  members  of  the  laminated 
structure. 

4.  When  a  laminated  structure  containing  both  quarter-sawed  and  plain-sawed  members 
is  subjected  to  conditions  which  cause  a  change  in  moisture  content,  the  unequal  swelling  or 
shrinkage  of  the  different  members  induces  stresses.     These  stresses  reach  a  maximum  and 
then,  if  the  moisture  content  remains  constant,  gradually  die  out.     The  structure  is  then  free 
from  internal  stresses  but  has  assumed  new  dimensions.     If  the  elastic  limit  of  the  wood  has 
not  been  exceeded,  the  strength  has  not  been  affected.     With  each  change  of  moisture  content 
new  stresses  will  be  developed. 

5.  When  a  laminated  structure  is  composed  of  members  all  plain-sawed  or  all  quarter- 
sawed  of  unequal  moisture  contents,  the  moisture  in  the  wood  tends  to  equalize,  and  stresses 
are  set  up  in  the  structure  through  the  unequal  shrinking  or  swelling  of  the  members.     These 
stresses  eventually  die  out,  leaving  the  structure  stress-free  but  with  changed  dimensions.     If 
the  elastic  limit  of  the  wood  has  not  been  exceeded  the  strength  has  not  been  affected.     If 
the  structure  is  subjected  to  further  moisture  change  no  stresses  are  induced,  since  all  members 
have  reached  the  same  condition  and  thereafter  act  together. 

6.  When  laminations  of  very  high  and  very  low  densities  are  glued  together  to  form  a 
laminated  structure,  change  of  moisture  content  induces  stresses  on  account  of  the  unequal 
shrinkage  or  swelling  of  the  members.     These  stresses  eventually  disappear;   and,  if  the  elastic 
limit  has  not  been  exceeded,  only  a  change  in  dimensions  results.     Further  changes  in  moisture 
content  induce  new  stresses.     Within  a  single  species  the  stresses  so  induced  are  relatively 
small,  however,  and  are  not  likely  to  be  serious  except  in  extreme  cases. 

7.  Animal  glue  used  in  these  tests  does  not  set  properly  when  the  laminations  are  of  high 
moisture  content.     The  exact  point  where  unsatisfactory  results  occur  can  not  be  determined 
from  the  data  available,  but  it  appears  to  be  between  15  and  18  per  cent.     Also,  in  glued 
specimens  placed  under  atmospheric  conditions  tending  to  produce  a  moisture  content  of 
from  15  to  18  per  cent  in  the  wood,  the  glue  softens  and  permits  the  laminations  to  be  easily 

pulled  apart. 

GENERAL  APPLICATION  OF  THE  INVESTIGATION. 

Warping  and  twisting  and  the  opening  of  glued  joints  are  of  great  importance  to  industries 
using  material  consisting  of  small  pieces  of  wood  joined  together  to  form  a  larger  structure. 
The  degree  to  which  such  changes  in  the  manufactured  products  are  detrimental  varies,  but  in 
many  cases  a  slight  change  is  sufficient  to  cause  rejection  or  at  least  necessitate  extensive  repairs. 

Ordinarily  the  furniture  industry  is  most  affected  by  such  failures,  and  when  furniture 
manufacturers  undertook  to  produce  airplane  propellers  on  a  commercial  scale  the  same  diffi- 
culties appeared  hi  a  magnified  form.  'The  smallest  changes  in  shape  or  track  and  any  opening 
of  glued  joints  were  reasons  for  rejection;  and  the  rejected  propellers  could  not  be  repaired  as 
could  articles  of  furniture. 

The  cause  of  warping  of  built-up  products  is  not  thoroughly  understood.  Several  factors 
are  commonly  credited  with  the  cause  of  most  failures,  and  these  may  appear  singly  or  in  com- 
bination. But  all  changes  of  form  or  opening  of  joints  are  the  result  of  the  development  of 
stresses  within  the  manufactured  article. 


INTERNAL   STRESSES   IN   LAMINATED   CONSTRUCTION. 


THE  PROBLEM  TO  BE  INVESTIGATED. 

Stress  is  defined  '  as  the  internal  force,  which,  when  a  body  is  subjected  to  external  forces, 
tends  to  hold  the  molecules  in  their  original  relation  and  to  preserve  the  integrity  of  the  body. 

Applying  external  loads  to  a  wood  structure  changes  its  shape  and  develops  proportionally 
resistant  stresses  until  the  elastic  limit  is  reached,  beyond  which  the  rate  of  deformation  increases 
until  rupture  occurs. 

Stresses  may  also  be  developed  in  wood  which  are  not  caused  by  external  loading,  but 
rather  from  conditions  within  the  wood.  While  they  probably  do  not  affect  the  mechanical 
properties  of  the  wood  fiber,  they  do  combine  with  loading  stresses  and  reduce  the  magnitude 
of  the  safe  external  load,  for  the  sum  of  both  stresses  can  not  exceed  the  strength  of  the  wood 
fiber.  Such  stresses,  caused  by  factors  other  than  external  loading,  are  properly  called  "  internal 
stresses"  and  are  important  becauses  of  their  influence  on  the  quality  and  strength  of  wood 
construction. 

Wherever  strength  properties  of  wood  are  involved,  internal  stresses  must  be  considered. 
In  the  seasoning  of  wood,  the  methods  and  rates  of  drying  and  the  quality  and  strength  of  the 
product  turned  out  of  the  kilns  depends  largely  upon  the  extent  to  which  the  magnitude  and 
character  of  internal  stresses  can  be  controlled. 

The  development  of  internal  stresses  is  due  largely  to  the  hygroscopic  properties  of  wood. 
Wood  contains  water  in  two  forms— as  free  water  in  the  cell  cavities,  which  is  given  off  first,  and 
as  moisture  absorbed  by  the  cell  tissues,  which  is  not  given  off  until  the  free  moisture  is  lost. 
The  point  at  which  moisture  begins  to  leave  cell  tissues  is  called  the  fiber  saturation  point. 
Below  this  point  wood  shrinks  with  loss  of  moisture  and  swells  with  gain  in  moisture,  coming 
to  an  equilibrium  with  every  climatic  condition  to  which  it  is  subjected  for  a  sufficient  length 
of  time.  Any  moisture  content  up  to  fiber  saturation  can  be  maintained  in  wood  by  proper 
control  of  the  temperature  and  relative  humidity  of  the  surrounding  atmosphere. 

The  magnitude  of  shrinking  and  swelling  with  moisture  changes  differs  not  only  for  every 
species  of  wood,  but  also  in  each  of  the  three  directions  in  a  tree — longitudinally  (along  the 
grain  of  the  wood,  radially  (along  the  radius  of  a  transverse  face),  and  tangentially  (along  the 
circumference  of  an  annual  ring).  Longitudinal  shrinkage  is  so  small  as  to  be  negligible  when 
compared  to  radial  and  tangential  shrinkage  values,  which  are  given  in  Table  1. 

TABLE  1. — Shrinkage  from  green  condition  to  oven-dry  condition. 


Species. 

Percentage  of  dimen- 
sions when  green. 

Radial. 

Tangential. 

Northern  white  oak  '                           

5.3 
3.9 
7.4 
5.2 
4.1 
4.8 
3.5 
4.8 
5.0 

9.0 
8.3 
9.0 
9.9 
6.9 
9.2 
4.2 
5.5 
5.7 

\ 

by  J.  A.  Newlin  and  T.  R. 


1  Bulletin  No.  556,  U.  S.  Department  of  Agriculture,  "Mechanical  Properties  of  Woods  Grown  in  the  United  States,' 
C.  Wilson. 

2  Results  of  more  recent  tests  made  at  Forest  Products  Laboratory,  Madison,  Wis. 

The  magnitude  of  the  shrinkage  across  the  face  of  a  board  varies  with  the  manner  of  cutting 
from  the  log.     Purely  quarter-sawed  lumber  (radial  face)  has  the  least  shrinkage.     Where 
unequal  shrinkage  or  swelling  with  moisture  change,  occurs,  as  in  boards  containing  I 
plain-sawed  and  quarter-sawed  material,  twisting  and  Clipping  result. 

In  laminated  construction,  the  danger  of  unequal  shrinking  and  swelling  i 
for  material  differing  widely  in  shrinkage  propertiesmay^combined,  and  smce 

i  Merriman's  "Civil  Engineers  Pocket  Book,"  1916  edition,  p.  272. 


REPORT   NATIONAL  ADVISORY   COMMITTEE   FOR   AERONAUTICS. 


f 

t 

t 

1 

t 

C 

T 

C 

i 

t 

1 

8 

«.— 

^  y_ 

-/£- 

3 
Q 

—  •> 

- 

can  not  swell  or  shrink  independently  (the  structure  must  change  as  a  whole)  the  excessive 
swelling  of  some  members  is  restrained  by  the  more  moderate  swelling  of  others,  equal  and 
opposite  stresses  being  developed  within  the  individual  members.  In  a  structure  so  stressed, 
the  internal  stresses  will  combine  with  loading  stresses  and  precipitate  failure  earlier  than  in 

a  structure  not  stressed.  A  comparison  of  the 
maximum  strength  of  two  structures,  one 
stressed  and  the  other  unstressed,  would  there- 
fore indicate  the  magnitude  of  internal  stress 
which  had  been  developed. 

The  influence  of  internal  stresses  on  the 
strength  of  wood  construction  is  of  particular 
importance  in  airplane  propellers,  where  maxi- 
mum strength  with  minimum  weight  and  perma- 
nency of  shape  are  prime  requisities.  This 
investigation  was  planned,  therefore,  to  cover 
those  sources  of  internal  stress  most  commonly 
encountered  in  the  manufacture  and  use  of  air- 
plane propellers. 

The  series  of  investigations  of  the  strength 
of  laminated  construction  includes  comprehensive 
tests  to  determine  2 — 

Series  A:  Influence  of  combining  plain-sawed 
;and  quarter-sawed  material. 

Series  B :  Influence  of  combining  material  of 
unequal  moisture  content. 

Series  C :  Influence  of  combining  high-density 
and  low-density  material. 


e 
—  i  — 

r_-- 

™.^r.t: 

-•-- 

t 

\ 

1 

1 

T 

C 

T 

\ 

1 

t 

- 

~Q 

_y-_ 

3" 
8 

"           72          * 

A. 


B. 


Fio.  1. — Combining  plain-sawed  and  quarter-sawed  material  in 
laminated  construction. 

A.— Swelling  of  laminated  construction  with  increase  in  moisture 
content. 

«=free  swelling  of  plain-sawed  member. 

o=  free  swelling  of  quarter-sawed  member. 
Since  members  are  glued  together,  they  must  swell  together  and 

final  swelling  =0+^^  (shown  by  fine  dotted  line).  This  develops 

t 

compression  in  plain-sawed  member  and  tension  in  quarter-sawed 
member. 

B. — Shrinkage  of  laminated  construction  with  loss  in  moisture 
content. 

«=free  shrinkage  of  plain-sawed  member. 
o=free  shrinkage  of  quarter-sawed  member. 
Since  members  are  glued  together,  they  must  shrink  together  and 
final  shrinkage =0+*^  (shown  by  fine  dotted  lines).    This  de- 
velops tension  in  plain-sawed  member  and  compression  in  quarter- 
sawed  member. 


METHOD  OF  INVESTIGATION. 

One  of  the  sources  of  internal  stress  is  the 
variation  in  shrinkage  properties  jn  different 
directions  in  a  tree,  the  effect  of  which  is  noticeable  in  combining  plain-sawed  and  quarter- 
sawed  material.  In  such  a  combination,  unequal  shrinking  and  swelling  tend  to  take  place 
with  moisture  changes,  and,  being  restrained,  cause  internal  stresses.  Figure  1  shows  the  char- 
acter of  stress  developed  with  a  change  in 
moisture  content  in  a  test  specimen,  such  as  is 
shown  in  figure  2. 

The  normal  free  swelling  of  the  plain-sawed 
faces  is  the  distance  "e,"  and  the  normal  free 
swelling  of  the  quarter-sawed  core  is  "  a . "  Being 
bound  together,  the  faces  are  restrained  and 
the  core  is  stretched,  developing  compressive 
stresses  in  the  faces  and  tensile  stresses  in  the 
core;  and  the  final  position  of  the  structure  is 
indicated  by  the  dotted  line.  A  loss  in  mois- 
ture results  in  stresses  of  opposite  character. 

In  either  case,  the  member  of  the  glued  specimen  subjected  to  internal  tensile  stress  will 
fail  under  a  smaller  external  load  than  if  it  were  free  from  internal  stresses.  After  such  failure 
the  entire  load  is  shifted  to  the  remaining  member,  and  complete  rupture  takes  place  at  a  com- 
paratively low  load.  The  whole  glued  structure  has  failed  then  under  an  external  load  smaller 
than  the  sum  of  the  loads  required  to  break  the  individual  free  members.3 


C 

J 

K 

P 

i 

"T 

T 

3" 
8 

-J"- 

3" 
5 

7? 

—  , 

—  . 

J:\L 

2  4 

3" 

~  4 

^ 

~'a~ 

\ection.  A  -/ 
r/ued  spec 

1 

imen 

t 

3 

s< 

Fret. 

~'a~ 

'ecfion  A-/ 
•  specimer 

FIG.  2.— Test  Specimen. 


* In  the  original  working  plan  (Appendix  B)  Series  A  is  designated  as  Series  I,  Series  B  as  Series  III,  and  Series  C  as  Series  II. 
1 "  External  load  "  and  "  strength  "  as  used  in  this  report  refer  to  test  conditions  such  as  are  obtained  in  the  test  shown  in  figure  6. 


INTERNAL   STRESSES   IN   LAMINATED   CONSTRUCTION.  5 

For  the  first  part  of  this  study,  specimens  similar  to  those  shown  in  figure  2  were  manu- 
factured, in  which  plain-sawed  and  quarter^sawed  material  were  combined,  moisture  and 
density  variables  being  eliminated.  Moisture  changes  were  introduced  to  develop  internal 
stresses  in  glued  specimens,  and  the  strength  at  test  was  compared  with  that  of  matched 
unstressed  specimens. 

The  second  source  of  internal  stress  investigated  was  the  unequal  shrinkage  developed  by 
gluing  together  laminations  differing  in  moisture  content.  All  wood  subjected  for  a  sufficient 
time  to  the  same  atmospheric  condition  will  come  to  practically  the  same  moisture  content.  If 
the  common  moisture  content  is  not  reached  before  assembly  into  laminated  construction,  it  is 
attained  after  assembly,  and  the  resultant  unequal  swelling  and  shrinking  of  the  component 


1 

t 

i 

\ 

C 

T 

C 

J 

\ 

1 

- 

3 
Q 

3" 
"~4 

J 
6 

•* 

—  /-  — 

lz 

-• 

FIG.  3. — Combining  material  not 
uniform  in  moisture  content  in 
laminated  construction. 

Dry  faces  and  wet  core  glued 
together  and  allowed  to  condition  to 
equilibrium. 

«=  free  shrinkage  of  wet  core. 

a=free  swelling  of  dry  faces. 
Final  position  shown  by  dotted 
line.  Since  members  are  glued 
together,  they  must  move  together. 
This  develops  compression  in  faces 
tending  to  swell  and  tension  in  core 
tending  to  shrink. 


t 

f 

t. 

1 

t 

c 

r 

C 

I 

t 

1 

3 
8 

*  

—  /—"- 

'  9 

3" 

5 

- 

e 

n;~ 

'-I- 

--_- 

t 

1 

1 

r 

C 

T 

\ 

\ 

\ 

-» 

3 
R 

~~4~~ 

3 

a 

~  /4"- 

* 

A. 


B. 


FIG.  4.— Combining  high  density  and  low  density  material  in  lami- 
nated construction. 

A. — Swelling  of  laminated  construction  with  increase  in  moisture 
content. 

Faces=  High -density  material. 
Core=  Low- density  material. 
e=free  swelling  of  high-density  member. 
o=  free  swelling  of  low-density  member. 

Since  members  are  glued  together,  they  must  swell  together  and 
final  swelling  =a-\ — =-  (shown  by  fine  dotted  line). 

This  develops  compression  in  high-density  member  and  tension  in 
low  density  member. 

B.— Shrinkage  of  laminated  construction  with  loss  in  moisture 
content. 

Faces=  High-density  material. 
Core=  Low -density  material. 
e=free  shrinkage  of  high-density  member. 
o=free  shrinkage  of  low-density  member. 

Since  members  are  glued  together,  they  must  shrink  together  and 
final  shrinkage  =0+^  (shown  by  fine  dotted  line). 

This  develops  tension  in  high-density  member  and  compression  in 
low-density  member. 

parts  develop  internal  stresses  as  shown  in  figure  3,  just  as  in  the  combining  of  plain-sawed 
and  quarter-sawed  material. 

Specimens  were  made  of  laminations  differing  in  moisture  content  at  assembly,  other 
variables  being-  eliminated,  and  these  specimens  were  conditioned  under  constant  atmos- 
pheric conditions  before  being  tested,  permitting  all  members  to  come  to  a  common  moisture 
content.  The  strength  of  internally  stressed  glued  pieces  was  then  compared  to  the  strength 
of  unstressed  free  pieces. 

The  third  source  of  internal  stress  investigated  was  the  combination  of  material  of  different 
densities.  High  density  wood  has  been  found  4  to  shrink  and  swell  more  than  low  density 
wood;  hence,  combining  material  of  different  densities  leads  to  the  development  of  internal 
stresses  with  moisture  changes  through  unequal  shrinking  or  swelling.  (See  fig.  4.) 

«  Bulletin  No.  676,  U.  S.  Department  of  Agriculture,  "The  Relatio^oftheShrinkage  and  Strength  Properties  of  Wood  to  its  Specific  Gravity, 
by  J.  A.  Newlin  and  T.  R.  C.  Wilson. 


6 


REPORT   NATIONAL  ADVISORY   COMMITTEE   FOR   AERONAUTICS. 


Specimens  were  manufactured  of  material  of  different  densities,  other  variables  being  elim- 
inated. Moisture  changes  were  introduced  to  develop  internal  stresses  and  the  strength  of  the 
glued  pieces  subjected  to  internal  stresses  was  compared  with  the  strength  of  the  free  unstressed 

specimens. 

MATERIALS  USED  IN  INVESTIGATION. 

LUMBER. 

Lumber  for  the  investigation  was  taken  from  the  stock  obtained  for  the  manufacture  of 
experimental  propellers.  It  was  handled  with  extreme  care,  and  all  pertinent  information 
concerning  the  particular  stock  was  obtained  and  recorded.  A  brief  description  of  the  material 
follows : 

CENTRAL  AMERICAN  MAHOGANY. 
AFRICAN  MAHOGANY. 
YELLOW  BIRCH. 

Part  of  the  material  of  each  species  was  purchased  in  the  form  of  logs  and  sawed  at  the 
laboratory.  The  remainder  was  sawed  at  outside  mills  under  laboratory  supervision.  All 
of  the  stock  was  kiln  dried  at  the  laboratory. 

NORTHERN  WHITE  OAK  (QUARTER-SAWED). 
NORTHERN  RED  OAK  (QUARTER-SAWED). 

Part  of  the  material  was  thoroughly  air-dried  stock  purchased  from  dealers.  The 
remainder  was  sawed  at  outside  mills  under  laboratory  supervision  and  kiln  dried  at  the 
laboratory. 

RED  GUM  (QUARTER-SAWED). 

This  stock  was  sawed  at  outside  mills  under  laboratory  supervision  and  kiln  dried  at  the 
laboratory. 

YELLOW  POPLAR. 

This  stock  was  purchased  in  log  form,  and  sawed  and  kiln  dried  at  the  laboratory. 
PHILIPPINE  MAHOGANY. 

This  material  was  from  War  Department  stocks  in  the  form  of  1-inch  kiln-dried  lumber. 

A  cutting  diagram  was  made  for  each  log 
sawed  at  the  laboratory  or  under  laboratory 
supervision  at  outside  mills.  Each  board  was 
numbered  for  future  identification,  and  these 
numbers  were  recorded  on  the  boards  and 
cutting  diagram.  A  sample  record  is  shown 
in  figure  5. 

The  kiln  drying  in  each  case  was  done  ac- 
cording to  specifications  for  propeller  stock. 

Upon  receipt  at  the  shop,  all  stock  was 
surfaced  and  stored  under  constant  conditions 
of  temperature  and  relative  humidity.  Sam- 
ples were  taken  from  both  ends  of  each  40-inch 
stick  for  use  in  making  density  determinations. 


End  view 


,  A/70/5 


End  view 


FIG.  5. — African  Mahogany. 


TYPE  OF  GLUE. 


The  glue  used  for  the  manufacture  of  the  laminated  specimens  was  an  animal  glue,  certified 
in  accordance  with  Bureau  of  Aircraft  Production  specification  No.  14000-A.  It  was  mixed 
in  the  proportion  of  1  part  of  glue  to  2£  parts  by  weight  of  water  and  heated  to  140  to  145°  F. 
before  being  applied. 


INTERNAL   STRESSES   IN   LAMINATED   CONSTRUCTION.  7 

SPECIAL  EQUIPMENT. 

The  same  shops  and  storage  rooms  were  used  for  carrying  out  this  investigation  as  were 
provided  for  the  propeller  manufacturing  and  storage  tests  which  are  being  conducted  at  the 
Forest  Products  Laboratory.3  In  these  rooms  the  temperature  and  relative  humidities  are 
constantly  maintained  at  the  following  values: 

Woodworking  room,  70°  F.,  with  55  per  cent  relative  humidity. 

Glueroom,    90°  F.,  with  65  per  cent  relative  humidity. 

Storage  room,  No.  3,  80°  F.,  with  30  per  cent  relative  humidity. 

Storage  room,  No.  2,  80°  F.,  with  60  per  cent  relative  humidity. 

Storage  room,  No.  1,  80°  F.,  with  90  per  cent  relative  humidity. 

SPECIMENS. 

The  test  specimens  used  were  the  standard  specimens  for  tension  across  the  grain,  having 
the  dimensions  shown  in  figure  2.  Each  test  piece  was  made  of  three  laminations.8  a  core 
f  inch  thick,  and  faces  I  inch  thick. 


PLATE  1. — Laminated  test  specimens  for  tension  across  the  grain,  showing  steps  in  manufacture. 

The  laminations  for  the  glued-up  and  free  (not  glued)  test  specimens,  shown  in  plate  1, 
were  matched  end  to  end  and  taken  as  near  each  other  as  possible.  Two  sticks,  A  and  B 
(about  40  inches  in  length),  carefully  selected  and  matched  for  density,  furnished  material 
for  test  specimens',  five  of  which  were  glued  and  five  not  glued,  or  free.  Stick  B  was  resawed 
longitudinally,  making  the  two  face  pieces,  Bt  and  B2.  The  40-inch  block  was  then  marked 
X  and  Y  as  shown  and  cut  in  two,  making  two  20-inch  blocks.  Block  X  was  then  glued  and 
kept  in  the  glue  press  24  hours.  The  gluing  operation  was  conducted  in  a  room  kept  under 
constant  conditions  of  temperature  (90°  F.)  and  relative  humidity  (65  per  cent).  The  lamina- 
tions of  block  Y  were  fastened  together  with  metal  staples.  The  marked  end  of  block  Y  was 
placed  opposite  the  marked  end  of  block  X  and  the  specimens  laid  out  and  numbered  as  shown. 
Odd  numbers  indicate  specimens  that  are  glued-up  and  even  numbers  those  not  glued.  The 
free  specimens  serve  as  a  standard  of  comparison  for  the  glued-up  specimens. 

6  A  complete  description  of  this  equipment  is  given  in  a  report,  "Automatic  regulation  of  temperature  and  humidity  in  an  experimental 
airplane  propeller  plant  and  its  application  to  commercial  conditions,"  by  A.  C.  Knauss,  June  2, 1919. 
•  In  some  of  the  latter  free  specimens  they  were  made  of  two  pieces  each  J  inch  thick. 

86540—22- 2 


REPORT   NATIONAL  ADVISORY   COMMITTEE   FOR   AERONAUTICS. 


METHOD  OF  TESTING. 

The  method  of  testing  these  specimens  is  indicated  in  figure  6,  which  illustrates  a  standard 
test  used  at  the  Forest  Products  Laboratory  for  the  determination  of  tensile  strength  across 
the  grain. 

The  selection  of  material,  marking,  care  of  specimens,  and  courses  of  conditioning  before 

testing  are  explained  in  detail  in  the  original 
working  plan,  a  copy  of  which  is  included  as 
Appendix  A  of  this  report. 

In  conditioning  specimens  they  were 
considered  at  equilibrium  with  the  constant 
conditions  in  which  they  were  stored,  when 
they  ceased  to  change  weight.  So  far  as 
moisture  content  is  concerned,  this  assump- 
tion is  correct,  but  from  a  standpoint  of 
stresses  induced  by  the  method  of  manufac- 
ture many  had  not  reached  their  ultimate 
condition.  Stresses  tend  to  die  out,  and  if 
all  specimens  had  been  allowed  to  remain 
in  any  one  condition  of  storage  for  an 
indefinite  period,  tests  would  have  shown 
them  to  be  stress  free.  This  fact  was  not 


fully  appreciated  in  the  beginning  of  this 
work,  and  care  was  not  taken  to  test  the 

specimens  immediately  after  reaching  apparent  equilibrium.     Sometimes  delay  was  necessary 

because  testing  machines  or  operators  were  otherwise  engaged. 


FIG.  6. 


RECORD  FORMS. 

Several  special  forms  were  used  in  recording  data.  A  sample  of  each  of  these  with  de- 
scriptive title  is  included  as  appendix  B  of  this  report. 

ANALYSIS  OF  RESULTS  OF  SERIES  A. 

THE  INFLUENCE  OF  COMBINING  PLAIN-SAWED   AND  QUARTER-SAWED   MATERIAL  ON   THE   STRENGTH  OF 

LAMINATED  CONSTRUCTION. 

The  following  species  were  studied : 
Central  American  mahogany. 
Philippine  mahogany. 
Yellow  birch. 
Yellow  poplar. 
Red  gum. 
Northern  red  oak. 

Specimens  were  made  with  plain-sawed  faces  and  a  quarter-sawed  core,  of  uniform  density 
and  conditioned  to  uniform  moisture  content  before  gluing.     After  manufacture,  the  specimens 
were  successively  subjected  to  several  atmospheric  conditions,  remaining  in  each  until  constant 
weight  was  reached.     Upon  leaving  each  condition  a  number  of  the  specimens  were  tested,  the 
remainder  passing  to  the  next  condition,  according  to  the  following  schedules: 
Schedule  No.  1 — Glue  room:  Room  No.  1,  room  No.  2,  room  No.  3. 
Schedule  No.  2 — Glue  room:  Room  No.  3,  room  No.  2,  room  No.  1. 

Relation  between  radial  and  tangential  tensile  strength  across  the  grain. — The  members  of  the 
free  specimens  were  tested  independently,  giving  separate  data  on  the  tensile  strength  across 
the  grain  of  the  plain-sawed  material  and  the  quarter-sawed  material.  The  average  of  the  unit 
strengths  of  the  individual  members  was  then  taken  as  the  strength  of  the  free  specimen.  The 
glue  specimens  were  necessarily  tested  as-  a  unit.  As  shown  in  figure  7,  the  tensile  strength 


INTERNAL   STRESSES   IN   LAMINATED   CONSTRUCTION. 


9 


across  grain  (across  the  face  of  the  board)  of  plain-sawed  material  is  designated  as  radial  tensile 
strength,  and  that  of  quarter-sawed  material  is  designated  as  tangential  tensile  strength,  on 
account  of  the  nature  of  the  failure. 

The  ratio  of  radial  to  tangential  tensile  strength  across  the  grain  obtained  in  this  investiga- 
tion, for  these  species,  is  shown  in  plate  2,  in  which  each  plotted  value  is  the  average  of  five  tests. 
This  ratio  seems  to  be  independent  of  moisture  content  up  to  20  per  cent,  but  varies  over  a 
comparatively  large  range  at  all  moisture  contents.  The  average  relations  found  for  each 
species  in  this  test  are  given  in  Table  2. 

TABLE  2. — Ratios  between  radial  and  tangential  strength  across  grain. 


Species. 

R 
T 

G 
T 

Q 
R 

Central  American  mahogany  

0.  73 

0  87 

1.  17 

Philippine  mahogany                     

.80 

90 

1.12 

Yellow  birch  

.81 

91 

1.11 

Yellow  poplar  

.70 

85 

1.22 

Red  gum  

.70 

84 

1.23 

Northern  red  oak  

.56 

78 

1.39 

R  =  unit  radial  tensile  strength  across  grain,  pounds  per  square  inch. 

T=  unit  tangential  tensile  strength  across  grain,  pounds  per  square  inch. 

O  =  unit  tensile  strength  across  grain  of  glued  specimens,  pounds  per  square  inch. 


\ 


x  Plane  of  failure^ 
Tangential  Radial 


These  ratios  indicate  that  plain-sawed  lumber  is  weaker  in  tension  across  the  face  of  the 
board  than  quarter-sawed  lumber,  particularly  in  red  oak.  The  medullary  rays  of  oak  are  very 
large  and  prominent,  and  checking  often  occurs  along 
these  rays  in  drying  lumber.  This  fact  may  account 

R 

for  the  extremely  low  ratio  of  ^  for  oak.  It  also  in- 
dicates how  easily  drying  may  reduce  the  radial 
strength  across  grain  of  plain-sawed  oak  lumber. 

Comparison  of  tensile  strength  across  grain  with 
changes  in  moisture  content. — Results  of  the  test  on 
series  covering  this  study  are  shown  in  plates  3  and  4. 
Ratios  of  maximum  unit  loads  carried  by  glued  speci- 
mens to  maximum  unit  loads  carried  by  free  speci- 


•irt 


111 


Test  for 
^  ^j-  tang  en  tial  tens  He 

mens  are  shown  at  values  of  p  ,  m,  and  are  plotted          strength 

K  +  J_  across  groin 


Test  for 
radial  tensile 

strength 
across  grain 


FIG.  7. 


against  change  in  moisture  content  after  gluing. 

2G 
It  will  be  seen  that  values  of  w^^r  do  not  always  equal  unity,  indicating  a  difference  in 


strength  between  glued  specimens  and  free  specimens.  This  may  be  due  either  to  the  presence 
of  internal  stresses  or  to  the  elastic  properties  of  the  wood.  If  internal  stresses  are  present, 
the  capacity  of  the  specimen  to  sustain  external  loading  is  ordinarily  reduced,  giving  a  ratio 
less  than  unity.  A  similar  ratio  is  also  obtained  if  the  elastic  properties  of  the  members  making 
up  the  glued  specimens  are  not  the  same.  The  strongest  member  receives  maximum  load  and 
fails,  thus  throwing  the  whole  load  on  the  remaining  members  and  producing  failure.  The 
total  load  which  the  piece  will  therefore.  support  may  be  less  than  the  combined  capacity  of  all 

the  members.     In  any  case,     S    could  not  exceed  unity  unless  the  glue  film  adds  strength, 


and  could  reach  unity  only  if  internal  stresses  were  so  distributed  that  the  maximum  strength 
of  all  members  is  reached  at  the  same  deformation,  which  would  be  rather  unusual. 


Heavy  lines  at  values  of  unity  for 


are  drawn  on  plates  3  and  4. 
2G 


mahogany,  Philippine  mahogany,  and  yellow  poplar,  the  values  of 


In  Central  American 
do  not  vary  far  from 


10 


REPORT   NATIONAL  ADVISORY   COMMITTEE   FOR  AERONAUTICS. 


Unit  strength  radio/  tension 

.8 
.7 

0 

c 

Q 

0 

0 

Q 

o 

o 

o 
o 

o 

O 

0 
o 

0 

0     C 
< 

o 

1 

o 

CENTRAL  AMERI 
MAHOGANY 

CAN 

t 

o 

0 

o 

0D 

o 

_ 

0 

0  0 

o 

o 

0 

0 

o 

3 

t 

.4 

a 

\ 

u 

e 

tf 

e 

e 

C 

Q 
••x.       j3 

e 

e 

a 
£ 

a 

o 
e 

c 

f 

^ 

HIL/PPII\ 
'AHOGAh 

<E 

\J\     .O 

o 

( 

•' 

e 

8, 

ee 

AJ 

iy 

.V7 

*C 

% 

ee 

o 

1 

t      c 

0 

O    ' 

o 

0 

0!          ° 

0 

n 

AO- 

0 

a 

o 
o 

p 

0°° 

0 

0 

( 

'EL  LOW 

BIRCH 

r 

>       7 

^^> 

O 

:^i^ 

00 

uo 
o 

0 
°0 

o 

0^ 

1 

1J 

.6 

\ 
0      -5 
k 

£      8 
I     .7 

!•* 

»>        /r 

^     .5 

5       •* 
* 

.7 
.6 

5 

.7 
.5 
.5 

.^ 

t 

0° 
0 

i 

1 

e 

e 

e 

e 

f 

t  ° 

a 

j 

a 

>n 

) 

• 

rz.z.< 

?W 

PDF 

'Z./4/ 

I* 

I    * 

e 

o 

e     s 

e 

e 
e 

•^rl 

e 

e 

— 

0 

°0 

fi 

0 

o 

0 

0 

! 

0 

fco 

0 

~~  F\ 

'ED 

GUf 

c 

>  <$ 

<D 

8 

o 

o 

' 

S 

o 
o 

o 
o 

7 

o 

0 

0 

e 

°& 

w  e 

o 

e 

s» 

Q 

X 

NORTHERN  RED  OAK 

«* 

^9° 

S 

e 

a«e. 

e 
a 

1 

/ 

1 

e 

o 

W° 

e 

L 

1                6                Q               IO                12              14               16               18              20 

Moisture  content  of  test  -  Per  cent 
PLATE  2.— Relation  between  radial  tensile  strength  across  grain,  tangential  tensile  strength  across  grain,  and  moisture  content. 


INTERNAL   STRESSES   IN   LAMINATED   CONSTRUCTION. 


11 


Unit  strength  glued  piece 

A\sf*r-/^rtf  *;+r-fi-ii->rhi  r-frf-ir\  = 

/£ 
t.4 
f.2 
1.0 
I.E 
I.O 
.8 
I.O 

.8, 

Q. 

!•' 

£  * 

S1 
|/, 

2w 
I'M 

I.O 

i 
•* 

I.O 
^ 

,      .8 

:  -6 

r\ 

.4 
\ 

1.2 
I.O 
1.2 
I.O 
.8 
1.0 
.8 
.6 

^4 

G             2G              G         G  -  Average  unit  strength  g/ucd  piece 
O=—      •  =  *  =  —        /?  =          ••             ••              ••          flat  -sawed  member. 
R             R+T                          7-.                          »               -quarter-        - 

0 

0 

o 

1 

> 
0 

c 

o 

I 
0 

i 

O 

0 

< 

0 

o 

o 

o 

• 

« 

i 
c 

i 

• 

• 

t 

• 

• 

I 

• 

( 

Jf/V, 

7?^  /Wf/f/ 
MAHOGANY 

CAf\ 

f 

• 

•    ' 

•   • 

• 

e 

• 
0    • 

• 
e 

t 

9 

i 

e 

1  •< 

# 

4 

e 

a 

« 

i 

• 

o 

Q 

O 

O 

1 

"  C 

n° 

O 

r 

O     O 

0 

o 

o 

~0"~ 

o 

o 

0  u 

o 

6 

c 

o 

• 

• 
• 

.  • 

• 

Pt 

ULIPPIM 

r 

•    4 

i 

• 

• 

.> 

• 
t 

• 
I 

• 

•  « 

•  • 

•  < 

»  ** 

M, 

WOt 

IAN, 

t 

Q 

o 

X" 

e 

»9 

e 

9 
O 

9 

e 

9 

e  < 

e 
4 

e 

«    « 

t 

e 

e 

e 

< 

)  

~"  "~  ~? 

0  0 

1 

o 

O 

j 

p 

O 

o 

0< 

0 

3  o& 
ov 

b 

0° 

J         C 

o      < 

) 

o 

0 

o 

t 

1 

0   • 

• 

• 

I 

_  i/ 

tu 

OW 

POF 

>LAfi 

i 

• 

i 

* 

( 

1 
• 

•   w 
• 
•  1 

P»» 

••  , 

t 

• 

r 

T 

< 

t 

*>* 

9 

a 

o 

•    e 

I 

a 

o 

•e  — 

< 

1 

1 

*   o 

<?< 

Le    ' 
l 

«    < 

e 

12  8  4  0  4  8  12 

Loss  Moisture  change  -  Per  cent  Gain 

PLATE  3.— Results  of  tests  showing  relation  between  tensile  strength  across  grain  of  laminated  specimens  (glued  of  plain-sawed  and  quarter-sawed 
material)  and  tensile  strength  across  grain  of  free  specimens  after  both  have  been  subjected  to  various  atmospheric  conditions. 


REPORT   NATIONAL  ADVISORY   COMMITTEE   FOR  AERONAUTICS. 


.  Unit  strength  glued  piece 

14 
1.2 
1.0 
.8 
1.0 
.8 
1.0 
.8 
.Q»  .6 

§. 

1" 

&Z 
£ 

S/.0 
ts 

*„ 

.8 

\     W 
:     .8 
* 

1      £ 

I     " 

* 

»    /2 

» 

^    1.0 
.8 
.6 
ft 
1.4 

i.a 

w 

a 

G              2G               G         G  ^  Average  unit  slrenqth  glued  piece 
O  =  —       •  =  «  =  —         R  -          -•              '•              ••           flat-  sawed  member 
R            R+T             T         T  =                        ••              ••  quarter- 

o0 

o 

o 

o 

<& 

00 

°< 

R°  ° 

o 

0 

0 

o 

o     c 

O 

0 

o 

• 

• 

• 

-  • 

•     • 

y. 

:LLt 

?M^ 

3//PC 

u 

• 
• 

• 

+ 

• 

••! 

f 

» 

V 

e 

~«» 

e 

* 

«• 
4 

fe   «, 

ae 

9 

e 

0« 

e     e 

yw 

o 

e 

e 

o 

c 

O  0( 

o 

00 

o 

Q 

o 

o 
o    o 

a 

o 

\ 

o 

u 
o 

o 

u 

• 

• 

^ 

SUfi 

A 

• 
• 

• 
• 

••• 

s 

• 

•  •  « 

•  • 

• 

••, 

• 

f\ 

1  " 

• 

—  a. 

& 

e 

»a 

e 

o 

a 

9 

0 

e 

e 

e«9 

5 

e 

e 

8  < 

e 

e 

W  w 

< 

1 

o 

o 

0 

O 

0 

a 

o 
o 

0 

0 

0 

c 

o 

• 

• 

• 

—  IUrt 

RTH 

*RN 

/?£D 

OA1 

's 

• 

• 

• 

•  • 

• 
• 

S 

e 

•• 

I 

• 

« 

• 

4 

a  9 

e  « 

e 

ee 

e 

o 

< 

> 

0 

o 

0 

• 

0 

u 

o 

VZ7A?i 

rHEt 

W/i 

^"Z? 

OAt 

• 

0 

• 

• 

o 

• 

• 
•  a 

• 

• 
9 

0 

.6 

6 

• 

e 

e 

0 

e 

4 

i 

8  4  O  4  8  12 

Loss  Moisture  change -Per  cent  Gain 

PLATE  4.— Results  of  tests  showing  relation  between  tensile  strength  across  grain  of  laminated  specimens  (glued  of  plain-sawed  and  quarter-sawed 
material)  and  tensile  strength  across  grain  of  free  specimens  after  both  have  been  subjected  to  various  atmospheric  conditions. 


INTERNAL   STRESSES   IN   LAMINATED   CONSTRUCTION.  13 

unity,  and  are  not  appreciably  affected  by  change  in  moisture  content  after  gluing.  In  the 
values  for  yellow  birch,  red  gum,  and  northern  red  oak,  however,  there  seems  to  be  a  reduction 
in  strength  as  losses  in  moisture  content  take  place  after  gluing.  This  inclination  can  not  be 
due  to  difference  in  elastic  properties  of  members  in  glued  pieces,  for  such  differences  would  be 
practically  the  same  for  all  changes  in  moisture  content.  It  is  more  likely  due  to  the  presence 
of  internal  stresses. 

The  specimens  of  plate  3  were  manufactured  and  conditioned  similar  to  those  of  plate  4. 
The  shrinkage  properties  of  all  species  are  similar  although  different  in  degree;  hence,  if  internal 
stresses  are  developed  in  one  species,  they  might  reasonably  be  expected  in  others,  and  the 
presence  of  internal  stresses  in  some  species  but  not  in  others  seems  inconsistent.  The  period 
of  conditioning  before  test,  however,  was  not  uniform  for  all  specimens.  Central  American 
mahogany,  Philippine  mahogany,  and  yellow  poplar  are  species  of  wood  which  change  moisture 
content  rapidly  and  reach  equilibrium  in  a  constant  atmospheric  condition  in  a  comparatively 
short  time.  The  species  in  plate  4  are  of  greater  density,  change  moisture  content  more  slowly, 
and  have  greater  radial  and  tangential  shrinkage  than  those  of  plate  3,  and  would  consequently 
develop  greater  stresses  with  moisture  changes. 

The  conditioning  data  show  that  specimens  of  yellow  poplar  and  Philippine  mahogany 
were  allowed  to  remain  under  constant  atmospheric  conditions  for  some  time  after  constant 
weight  had  been  reached,  being  tested  after  periods  of  13  to  25  days  in  the  final  conditioning 
room.  The  values  of  0.91  and  0.93  at  moisture  losses  of  10.8  amd  9.5  per  cent  in  Central 
American  mahogany  are  from  specimens  tested  after  a  period  of  only  five  to  seven  days,  and 
the  periods  for  values  shown  on  plate  11  range  from  five  to  eight  days.  Apparently,  where 

2G 
ratios  of  „  .  ™  have  fallen  off,  indicating  the  presence  of  internal  stresses,  the  specimens  were 

tested  after  having  been  subjected  to  climatic  change  for  only  a  short  period,  while  in  those 
permitted  to  condition  under  uniform  atmospheric  condition  for  longer  periods  before  test, 
internal  stresses  were  not  present.  Since  the  species  for  which  these  ratios  showed  internal 
stress  require  longer  periods  to  reach  equilibrium  with  climatic  changes  than  the  species  for 
which  the  ratios  showed  no  internal  stress,  the  results  indicate  that  the  magnitude  of  internal 
stresses  changes  with  time.  The  internal  stresses  are  set  up  as  swelling  or  shrinkage  takes 
place,  which  in  turn  depends  on  the  change  in  moisture  content.  After  constant  weight  is 
reached,  however,  further  stresses  are  not  set  up,  and,  judging  from  the  results  of  this  test, 
those  already  set  up  seem  to  die  out. 

Had  the  specimens  for  values  in  plate  4  remained  in  conditioning  rooms  for  longer  periods 
before  test,  their  ratios  would  no  doubt  have  approached  unity,  and  if  allowed  to  remain  for 
comparatively  long  periods,  would  probably  have  equaled  the  ratio  of  specimens  in  which  no 
moisture  change  took  place. 

The  results  of  these  tests  do  not  permit  us  to  study  the  rate  of  change  of  internal  stress, 
either  while  in  development,  or  while  dying  out.  Neither  can  the  maximum  internal  stress 
developed  be  determined  from  these  results,  for  some  tests  were  perhaps  made  while  stresses 
were  not  fully  developed,  and  in  others  the  stresses  at  the  time  of  test  had  already  fallen  below 
the  maximum.  Maximum  internal  stresses,  however,  would  in  all  cases  be  as  great  or  greater 
than  any  shown  in^this  study. 

Relation  between  tensile  strength  across  grain  of  Hocks  made  of  all  quarter-sawed  boards  as 
compared  to  blocks  of  all  plain-sawed  boards.— The  relation  between  the  tensile  strength  across 
grain  of  flat-sawed  and  of  quarter-sawed  material  has  already  been  given  in  Table  2.  The 
tensile  strength  across  grain  (across  the  face  of  the  board)  of  quarter-sawed  material  being  greater, 
laminated  stock  of  quarter-sawed  material  will  develop  the  greatest  strength  in  the  direction 
of  glue  joints,  and,  because  of  its  lesser  shrinkage,  will  develop  smaller  internal  stresses,  acting 
parallel  to  the  glue  joints.  Purely  plain-sawed  constructions  produces  the  weakest  structure 
in  the  direction  of  the  glue  joints. 


14  REPORT   NATIONAL  ADVISORY   COMMITTEE   FOR  AERONAUTICS. 

The  maximum  strength  that  can  possibly  be  developed  in  laminated  construction  is  ob- 
viously the  sum  of  the  maximum  strengths  of  the  individual  members.  Where  plain-sawed 
and  quarter-sawed  material  are  both  used  in  the  laminated  construction,  the  maximum  unit 

Tf  -4-  T          ^C1 
strength  G  occurs  when  it  equals  — ~ — '  or  p  ,  m=  1 .    Using  this  maximum  strength  value  of  G  as 

1,  the  maximum  unit  strength  of  laminated  construction  containing  both  plain-sawed  and  quarter- 
sawed  material  compared  to  the  unit  strength  of  purely  quarter-sawed  construction  becomes 
n,  'T  n  (} 

-~=-=j1-     Values  of  ™from  the  test  are  shown  in  Table  B,  and  plotted  in  plates  10  and  11. 

This  value  was  always  found  to  be  less  than  unity. 

C1         C1 
Values  of  ^  and  „  when  G  =  I  are  shown  as  dotted  lines  in  plates  3  and  4.     In  the  specimens 

rt 

made  of  northern  red  oak,  values  of  „  were  more  variable  when  conditioned  from  a  high  mois- 
ture condition  directly  to  a  dry  condition  (lower  set  of  data  for  northern  red  oak,  plate  4)  than 
when  conditioned  from  a  high  to  a  low  moisture  content  by  successive  steps  (upper  set  of  data 
for  northern  red  oak,  plate  4) .  This  is  perhaps  due  to  the  ease  with  which  this  species  checks 
along  the  medullary  rays  in  rapid  drying.  When  once  formed,  these  checks  permanently 

reduce  the  radial  tensile  strength  of  the  wood,  for  although  swelling  may  again  close  the  checks 

ft 
and  make  them  invisible  the  loss  in  strength  is  permanent.     Values  of  m  are  not  so  affected 

since  incipient  shakes  occur  less  frequently  in  this  direction. 

CONCLUSIONS  FROM  SERIES  A  TESTS. 

From  this  series  of  tests  it  is  concluded  that — 

1.  Tensile  strength  across  grain  (across  the  face  of  the  board)  for  quarter-sawed  lumber  is 
greater  than  for  plain-sawed  lumber.     Plain-sawed  lumber  may  be  from  20  to  50  per  cent 
weaker  across  the  grain,  depending  upon  the  species  and  method  of  drying. 

2.  The  gluing  together  of  plain-sawed  and  quarter-sawed  stock  gives  rise  to  the  develop- 
ment of  internal  stresses  through  unequal  swelling  and  shrinkage  with  changes  in  moisture 
content,  and  results  in  a  weakening  across  grain  of  the  laminated  structure. 

3.  When  a  laminated  structure  containing  both  quarter-sawed  and  plain-sawed  members 
is  subjected  to  conditions  which  cause  a  change  in  moisture  content,  the  unequal  swelling  or 
shrinkage  of  different  members  induce  stresses.     These  stresses  reach  a  maximum  and  then 
gradually  die  out.     The  structure  is  then  free  from  internal  stresses  but  has  assumed  new  dimen- 
sions; and,  if  the  elastic  limit  of  the  wood  has  not  been  exceeded,  the  strength  has  not  been 
affected.     With  each  change  of  moisture  content  new  stresses  will  be  developed. 

ANALYSIS  OF  RESULTS  OF  SERIES  B. 

THE  INFLUENCE  OF  COMBINING   MATERIAL  OF   UNEQUAL  MOISTURE  CONTENT  ON  THE  STRENGTH  OF  LAMINATED 

CONSTRUCTION. 

Test  specimens  for  this  series  of  tests  were  made  of  the  following  species,  as  outlined  in  the 
working  plan: 

Central  American  mahogany. 
Philippine  mahogany. 
Hard  maple. 
Yellow  poplar. 
Yellow  birch. 
African  mahogany. 
Northern  white  oak. 
Northern  red  oak. 
Red  gum. 


INTERNAL   STRESSES   IN   LAMINATED   CONSTRUCTION. 


15 


Specimens  were  manufactured  in  which  the  moisture  content  of  the  core  differed  from  that 
of  the  facets  by  various  amounts.  The  material  for  any  species  was  either  entirety  plain  sawed 
or  entirely  quarter  sawed  and  within  specimens  was  uniform  in  density.  Specimens  as  before 
described  were  subjected  successively  to  various  atmospheric  conditions  before  test. 

The  properties  of  hide  glue  were  found  to  affect  materially  some  of  the  results  of  this  test. 
Hide  glue  is  hygroscopic,  and  its  strength  varies  rapidly  with  moisture  changes.  When  exposed 
to  a  90  per  cent  relative  humidity,  this  glue  softens  until  it  has  very  little  strength,  and  glue 
joints  open,  as  shown  in  plate  5.  Plate  (>  shows 
the  result  of  attempting  to  glue  together  lami- 
nations of  red  oak  at  18  per  cent  moisture  con- 
tent. Sufficient  water  did  not  leave  the  glue  to 
permit  it  to  set  and  develop  its  full  strength. 

In  this  series  the  specimens  glued  at  18  per 
cent  moisture  content  showed  no  adhesion  until 
the  moisture  content  of  the  wood  had  been 
reduced  to  the  point  of  setting.  By  this  time, 
the  dry  members  of  the  series  had  absorbed 
moisture  and  the  actual  difference  in  moisture 
content  between  core  and  faces  at  the  time  when 
the  glue  was  able  to  transmit  stress  was  much 
less  than  when  the  pieces  were  first  assembled. 
The  development  of  internal  stress  in  these 
specimens  would  therefore  correspond  to  mois- 
ture difference  when  the  glue  had  set  enough 
to  transmit  stresses,  rather  than  to  the  original 
moisture  difference. 

In  the  conditioning  of  specimens,  this  factor 
again  appeared.  Upon  entering  an  atmosphere 
of  90  per  cent  relative  humidity,  the^glue 
softened  within  a  few  days,  and  members  of 
specimens  under  stress  were  permitted  to  move 
over  each  other,  thereby  relieving  the  stress. 
The  initial  stress  having  disappeared  and  no 
"further  source  of  internal  stress  being  present, 
all  subsequent  tests  would  show  complete 
regain  in  strength. 

HARD  MAPLE  (pi.  7). 

Specimens  of  this  species  were  made  of 
plain-sawed  material.  Those  glued  with  mois- 
ture differences  of  11  per  cent  began  to  check 
and  split  open  soon  after  gluing,  as  illustrated 
in  plate  8.  The  shrinkage  of  the  wet  member 
and  the  swelling  of  the  dry  member  in  coming 
to  a  common  moisture  content  were  so  rapid 
in  these  specimens  that  stresses  beyond  the 
strength  of  the  wood  were  developed  and  failure  resulted.  Specimens  similarly  manufactured 
which  did  not  check  and  split  open  showed  low  strength  ratios  upon  being  tested  from  the  glue 
room,  but  after  being  conditioned  in  room  1  or  3,  showed  a  considerable  regain  in  strength,  and 
continued  to  regain  strength  with  further  conditioning.  Similar  results  were  obtained  on 
specimens  glued  at  smaller  moisture  differences,  indicating  that  the  magnitude  of  the  internal 
stresses  was  decreasing  with  continued  conditioning.  In  practically  all  cases,  specimens 
conditioned  in  a  high  humidity  showed  remarkable  regain  in  strength,  due  probably  to  softening 
of  the  glue  and  consequent  release  of  stresses. 
86540—22 3 


PLATE  5.— Showing  failure  of   glue  joints  when  subjected  to  high 
humidity  conditions. 


16 


REPORT   NATIONAL  ADVISORY   COMMITTEE   FOR  AERONAUTICS. 


Results  of  tests  on  specimens  glued  at  uniform  moisture  content  were  quite  uniform, 
indirating  that  where  stresses  are  not  developed,  variations  for  the  species  are  quite  small. 
The  elastic  properties  of  members  of  glued  specimens  being  quite  similar,  no  variation  in  strength 
ratios  would  be  expected  from  this  source. 
PHILIPPINE  MAHOGANY  (pi.  0). 

Specimens  of  this  species  were  made  of  plain-sawed  material.  Results  of  these  tests  were 
much  more  uniform  than  in  maple  and  no  failures  directly  after  gluing  were  recorded.  Even 
with  initial  moisture  differences  of  8  and  11  per  cent,  specimens  tested  from  the  glue  room 
showed  a  maximum  of  only  10  to  20  per  cent  reduction  in  strength,  and  this  was  reduced  by 
further  conditioning  until  it  fell  within  the  strength  variations  for  this  species.  As  in  the  series 


PLATE  6.— Showing  glue  joint  failures  (northern  white  oak).    Pieces  glued  at  18  per  cent  moisture  content  and  held  under  pressure  for  24  hours. 

of  plain-sawed  and  quarter-sawed  specimens,  a  comparatively  longer  period  of  conditioning  was 
given  specimens  of  Philippine  mahogany,  permitting  reduction  in  internal  stress  to  take  place 
before  test. 

CENTRAL  AMERICAN  MAHOGANY  (pi.  10). 

Specimens  of  this  species  were  made  of  plain-sawed  material.  This  species  changes  moisture 
content  rapidly,  and  its  radial  tensile  strength  across  grain  is  low  and  somewhat  variable,  resulting 
in  a  large  variation  in  strength  ratios,  even  when  the  specimens  were  glued  at  uniform  moisture 
content.  In  specimens  glued  with  moisture  differences  between  face  and  core,  results  were  more 
variable,  regain  in  strength  being  found  in  some  specimens  and  apparent  loss  in  strength  being 
found  in  others. 

There  seemed  to  be  a  general  inclination  toward  higher  strength  ratios  at  low  moisture  con- 
tents. If  the  glue  film  adds  any  strength,  its  effect  on  the  strength  ratio  would  be  most  apparent 
in  species  of  low  tensile  strength,  across  grain  and  at  low  moisture  contents,  at  which  glue  has 
its  greatest  tensile  strength.  This  may  be  partly  responsible  for  such  inclination  in  this  species. 
YELLOW  BIRCH  (pi.  11). 

Specimens  were  made  of  plain-sawed  naterial.  Moisture  changes  take  place  comparatively 
slowly  in  this  species,  and  the  radial  tensile  strength  across  the  grain  is  comparatively  high. 


INTERNAL  STRESSES  IN  LAMINATED  CONSTRUCTION. 


17 


333id  pan/6  u^6  us^s-  //o/) 

1.2 
1.0 
.8 
.6 
12 
1.0 

.Q 

o 

Q. 

*»   ^ 
<li  j4 

4s 

IP 
> 

>. 
;    .e 

i 

i»    •' 
* 

!    .2 

V 

i 
:      i.O 

.8 
.6 
1.2 
1.0 
.8 
.6 

.4 

y 

/JH/P0  /W££ 
O  Tested  offer  g 
lo/rtroom  No.  1          ©         ••            ••     cc 

9  Tested  offer  gluing 
O         ..            »      conditionir 
© 

fuing 
•mentioning  in  room  No.  3 
362 
3,2  Ql 

s 

^ 

)«a 

9 

a 

O 

•  * 

| 

^   ®w 

P 

O 

o 

©  © 

o 

e 

e 

» 

e 

a 

e 

e 

6      e 

03 

e 

9 

A 

loisfure  < 
Lominc 

difference  of  time  of  'o/u/n< 
ted  construction  =  O.O  °70 

1 

A 

toisiure  difference  of  time  ofgluin 
Laminated  construction  =  3.O  °70 

9 

+ 

0 

*     e 

i 

>     © 

o 

• 

0 

O 

•* 

j        < 

I 

e 
e    e 

e 

?     ft 

*  e« 

9 

. 

I 

e 

e 

e 

9 

e 

9 

/ 

Moisture  difference  oft/me  ofg/uin 
L  aminafed  construction  =4.0°7o 

9 

A 

foisfure  difference  at  time  ofg/u/rx 
Laminated  consf  ruction  =  7.0°7o 

7 

0 

t 

I 

o 

U    © 

o 

e 

» 

3 

o> 

( 

) 

i 

» 

e 

e 

« 

e 

& 

A 

/o/s/ 
Lor 

jre 
rtinc 

Jiffe 
fed 

renc 
cons 

•e  of 
frut 

//>T7€ 

://b/- 

•  of  t, 
=-ff.t 

luin 
1°70 

7 

A 

1oist 
Lot 

e 

C//"6> 

nine 

diffe 
ited 

renc 
con: 

~e  or 
•frui 

timti 
?tior 

»<?/"$ 
>-//.< 

>/C//>7 

9%' 

? 

12 


16 


20 


ie          20  4  e  12 

Moisture  content af  tesf  -  Per  cenf 

PLATE  7. — Results  of  tests  showing  relation  between  tensile  strength  across  grain  of  laminated  specimens  (glued  at  uniform  and  at  nonuniform 
moisture  conditions)  and  tensile  strength  across  grain  of  free  specimens  after  both  have  been  subjected  to  various  atmospheric  conditions. 


18 


REPORT   NATIONAL,  ADVISORY   COMMITTEE   FOR  AERONAUTICS. 


Consequently,  strength  ratios  were  more  uniform.  In  specimens  manufactured  with  large 
moisture  differences  no  great  development  of  internal  stress  appeared  when  tests  were  made  on 
them  directly  on  leaving  the  glue  room,  and  further  conditioning  brought  practically  all  results 
within  the  strength  variation  for  this  species. 

YELLOW  POPLAR  (pi.  12). 

Specimens  were  made  of  plain-sawed  material.  This  species  is  easily  affected  by  moisture 
changes.  Results  from  other  species  indicated  that  moisutre  differences  of  3  per  cent  were 
practically  negligible ;  hence  this  condition  was  omitted  in  this  test.  As  shown  in  the  series  on 

plain-sawed  and  quarter-sawed  specimens,  the 
study  on  yellow  poplar  was  extended  over  a 
comparatively  long  period  of  time.  This  was 
not  intentional,  but  was  due  to  unforeseen 
delays  in  testing.  Results  of  tests  for  this 
species  gave  little  indication  of  stress  at  time 
of  test,  showing  that  if  stresses  had  developed, 
they  had  disappeared  before  test  to  such  an 
extent  that  strength  ratios  fell  within  the 
strength  variation  for  the  species. 

AFRICAN  MAHOGANY  (pi.  13). 

Specimens  were  made  of  quarter-sawed 
material.  This  species  changes  moisture  con- 
tent readily.  Strength  ratios  for  specimens 
glued  with  large  moisture  differences  were 
only  slightly  lower  than  for  those  glued  at 
uniform  moisture  content,  and  were  a  maxi- 
mum when  tested  at  low  moisture  contents, 
again  suggesting  the  possibility  of  glue-film 
strength  affecting  the  ratio.  Only  slight  indi- 
cations of  internal  stress  were  found. 

RED  GUM  (pi.  14). 

Specimens  were  made  of  quarter-sawed 
material.  Tangential  tensile  strength  across 
the  grain  is  comparatively  high,  and  moisture 
change  takes  place  at  a  moderate  rate. 
Strength  ratios  of  specimens  glued  at  large 
moisture  differences  showed  as  much  as  20  per 
cent  reduction  in  strength,  but  in  nearly  all  cases  this  was  reduced  by  further  conditioning, 
indicating  the  dying  out  of  stresses. 
NORTHERN  WHITE  OAK  (pi.  15). 

Specimens  were  made  of  quarter-sawed  material.  This  species  changes  moisture  content 
slowly,  and  possesses  high  tangential  tensile  strength  across  grain,  giving  rather  small  variation 
in  strength  ratios.  Only  slight  indications  of  internal  stress  at  time  of  test  were  found,  and  in 
such  cases  further  conditioning  served  to  reduce  the  magnitude  of  the  internal  stress.  No 
doubt  if  the  specimens  had  been  tested  immediately  after  reaching  moisture  equilibrium, 
greater  stresses  would  have  been  found. 
NORTHERN  RED  OAK  (pi.  16). 

Specimens  were  made  of  quarter-sawed  material.  Tangential  strength  across  grain  is 
comparatively  high,  resulting  in  a  small  strength-ratio  variation,  and  moisture  changes  take 
place  slowly.  Specimens  were  tested  after  conditioning  in  the  glue  room  only.  Indications 
of  internal  stress  were  slight,  even  for  specimens  manufactured  with  high  moisture  differences. 
This  was  probably  due  to  the  long  conditioning  period.  Further  conditioning  served  to  reduce 
the  magnitude  of  internal  stresses  until  they  fell  within  the  strength  variation  for  the  species. 


PLATE  8.— Laminated  marie  showing  tenskn  failures  due  to  moisture 
difference  at  time  of  gluing.  Odd  numbers  indicate  glued  pieces. 
Even  numbers  indicate  -free  pieces. 


INTERNAL   STRESSES   IN   LAMINATED   CONSTRUCTION. 


19 


(Jnif  sfrengfh  glued  piece 

1.2 
1.0 
.8 
.6 
1.2 
1.0 

.8 

<n 

.2.5 

CL 

4J      ., 

1* 

^1.2 
C* 
v/.O 
>; 
^.8 

^ 

1.2 
i 

3    i-o 

\      M 
> 

5      .6 

« 

0 

u      .4 

> 

3 

B     -2 
1    ,.0 

.8 
.6 
1.2 
1.0 
.8 
.6 
.4 

.2 

* 

PHILIPPINE:  MAHHGANY 

d  Tested  after  gluing                                                9  Tested  after  gluing 
Q        .,            ••      conditioning  in  room  No.l          e         "            "     conditioning  in  room  No.  3 
®       »              <•        ••        -I&2       ®        '•            "                 ••             ~       -       "3&2 
•       -                                          /.2&3      ®        "           ••                3.2&I 

0 

• 

•  • 

• 

• 

• 

D 

o 
o 

« 

O 

t 

e 

e 

e 

<§ 

^ 

® 

e 
e© 

A 

loisture  difference  at  time  of  gluing 
Laminated  construct/on  =  0.0°70 

A 

Moisture  difference  ottime  ofgluini 
Laminated  construction^  3.O°70 

7 

• 

• 

e 

O 

• 

2 

• 

*»^ 

® 
? 

O 

• 

*te 

H 

e~e> 

- 

(D 

e 

9  ' 

b 

l« 

®      ® 

**< 

« 

$ 

A 

•foisture  difference  offime  ofg/uin 
Laminated  construction  =4.O°7o 

9 

A 

loisture  difference  at  time  ofg/uint 
Laminated  construction  -  7.O°7o 

7 

• 

• 

• 

•e 

@ 

® 

o 

0 

• 

«  e 

© 
e 

o 

e 

e 

e 

e 

e 

o 

til 

e 

> 

®0ee 

€ 

A 

loisture  difference  of  time  ofgluin 
L  aminafed  cons  true  fion  ~  8.  0  °70 

9 

/i 

ioisture  difference  at  time  ofgluini 
Laminafed  construction-  1  1.O^To 

7 

7                8                12               16              2O               4                8                12              16               20 

Moisture  contenfaf  -test  ~  Per  cent- 
PLATE  9. — Results  of  tests  showing  relation  between  tensile  strength  across  grain  of  laminated,  specimens  (glued  at  uniform  and  at  nonuniform 
moisture  conditions)  and  tensile  strength  across  grain  of  free  specimens  after  both  have  been  subjected  to  various  atmospheric  conditions. 


20 


REPORT   NATIONAL  ADVISORY   COMMITTEE   FOR  AERONAUTICS. 


CENTRAL  AMERICAN  MAHOGAN 
e  Tested  after  q 
>ginroomNo.f         e       "           "     cc 

'Y 

O  Tested  aftergluina. 
©        ••            "      conditionir 

'uing 
ndirioning  in  room  No.  3 

1.2 

. 

9 

® 

o 

• 

\ 

1.0 

% 

< 

W 

e 

• 

e 

9 

c 

o 

.8 

0 

Unit  strength  glued  pfece 
Average  strength  ratio  «  —  —  :  —  — 
Unit  strength  free  piece 

K)  '-^  6)  CD  ^  r\>  bj  Cb  '^  h»  X  &>  CD  cs  K»<h  bo  QJ  K»'^  6>  oo  b  K 

« 

j 

-v— 
e 

1 
9 

>e§ 

A 

e 
A 

9 

. 

e 

X 

A; 

oistL 
Lan 

ire  c 
tino 

iiffet 
fed  i 

•tf/7C 

rons 

e  o// 
true 

(•/me 
tion 

°ogc 

%™ 

r- 

A 

lot's  t 
Lar 

jre 
nine 

y/'/fe 
ifed 

renc. 
cons 

-e<7/ 
/^ro-( 

///77€ 

://bn 

'  ofc, 
=  J"." 

r/u/n< 
?°70 

7 

• 

o 

V 

*© 

< 

<D 
I 

„ 

9 

o 

* 

9 

• 

\ 

t 

e 
e 

e  ] 

e 

\j 

0 

* 

e 

e 

e 

te 

A 

'oist 
Lar 

jre  < 
r?y/7C 

rt/ife 
'A?c/ 

-e/TC 
CO/7J 

•ec??1 

/T4A 

time, 
-tion 

•  ofg 
=  4.1 

/u/'n 
?°70 

7 

A 

foist 
Lot 

jre 
nine. 

^///e 
ited 

renc. 
com 

-eat 
•trui 

///T?^ 
r/vbr 

'  ofc, 
>=  7. 

'luin 

0°lo 

7 

, 

0 

o 

* 

e   e 

• 

9 
9 

o 

© 

* 

% 

0 

0 

e 

j 

e 

9     « 

e 

e 

e 
e 

e 

* 

e 

9 

A 

foist 
Lai 

ure 
nine 

://Yfe 

r/ffo' 

/-e/7« 
com 

:ea/ 
;?V-t/( 

timt. 
•:t/or 

"ofc, 
'  =  8. 

iluin 
0°7o 

^ 

A 

1oist 
Lar 

e 
^/-£> 

77//7C 

diffe 
ifed 

renc 
com 

:e  of 
•frui 

fim€ 
-tior 

'of<i 
<=//.i 

i/uin 
?°7o 

•j 

4  8  ia  16  ^0  4  Q  /e  J6  20 

Moisture  content  of  test  -  Per  cent 

PLATE  10. — Results  of  tests  showing  relation  between  tensile  strength  across  grain  of  laminated  specimens  (glued  at  uniform  and  at  nonuniform 
moisture  conditions)  and  ten$i  c  strength  across  grain  of  free  specimens  after  both  have  been  subjected  to  various  atmospheric  conditions.    ^ 


INTERNAL   STRESSES   IN   LAMINATED   CONSTRUCTION. 


21 


Unit  strength  glued  piece 

1.2 
1.0 
.8 
.6 
1.2 
1.0 
.8 

<U 

.<"  .6 
Q. 

(U      . 
Q)   .4 

1 

£•* 
O 

via 

•i; 

•0 

-t;  .<S 

S, 

1.2 
> 

i  //? 

;   .8 
» 

i    .6 
I   A 

1  /.2 
0 

.8 
.6 
1.2 
I.O 
.8 
.6 
.4 
.2 

YELLOW  BIRCH 

9  Tested  after  g 
•join  room  No./          ©        "             "     c< 
I&2       @ 

-/.2a3     e      " 

0  Tested  after  g/uing 
O         "            "      condition// 
® 
• 

luing 
•)nd/t/'oning  in  room  Mo.3 
3&2 
3.B&I 

• 
• 

®  ® 

00 

C 

0 

• 

°f> 

i 

r-^J- 

O 

i 

o 

• 

e 

e 

rfl 

© 
© 

»      © 

e 

% 

e 

[ 

5    OO 

• 
e 

e 

e 

e 

e 

A 

loisfure  difference  aftime  of  gluin 
Laminated  construction  =  0.0  'Jo,  ' 

? 

/» 

1o!sture  difference  at  time  of  glu/r* 
Laminated  construction  *3.Q°f0 

9 

•% 

3      ® 
®» 

u 

• 
• 

l 

® 

e 

O 

o 

V 

w 

a 

e 

( 

>\e 

9 

e 

e 

t) 
% 

e 

e   * 

® 

< 

i 

e 

I 

i 

e 

e 

_A 

loist 
La/ 

ure  < 
~n/nc 

y/y/e 

ited 

9 

ren< 
com 

:e<?/ 

i/'/'O' 

ti'mt 
cffbr 

?  ofc 
1=4. 

f/uin 
?7o 

7 

A 

•toisf 
Lot 

ure 
"nine 

diffe 
ifed 

ren<. 
com 

reoT* 
trui 

///T7€ 

-t/or 

of  f, 
=  7.< 

?/c///> 

7% 

?  

® 

t 

0 

e 

0 

i 

»  • 

Q® 

0  0 

• 

• 

©^ 

f 

e 

e 

e 

' 

9 

9 

e 

e 

e 

e 

(!) 

A 

lotsture  difference  at  time,ofgluin<. 
Laminated  construction  =  8.O  °70 

7 

A 

Moisture  difference  at  fime  of  gluing 
Laminated  cons  t  ruction  *l/.O°7o 

? 

4               5               /2              /6             BO              4               8/2              16             BO 

Moisture  content  at  test  -  Per  cenf 
PLATE  11. — Results  of  tests  showing  relation  bet  veen  tensile  strength  across  grain  of '.aminated  specimens  (glued  at  uniform  and  at  ncnuniform 
moisture  conditions)  and  tensile  strength  across  grain  of  free  specimens  after  both  have  been  subjected  to  various  atmospheric  conditions. 


22 


REPORT   NATIONAL  ADVISORY   COMMITTEE   FOR   AERONAUTICS. 


8 


Unit  strength  glued  piece 

1.0 
.8 
.6 
1.2 
1.0 

.8 

<j 
•8!  .6 
Q. 

1.4 
•J; 

«> 
-^ 

1.2 
} 

\     1.0 

:      .8 
ft 

3     .5 
i 

D        .4 
>> 

J       •* 

\ 

:       /.0 

.<9 
.6 
1.2 
/.O 
.8 
.6 
.4 

YELLOW  POPLAR 

O  Tested  after  gluing                                                  *  Tested  after  g/uing 
O        ••            '•      conditioning  in  room  No.  1           e         ••             "     conditioning  in  room  No.  3 
©        .,            ..                  ..             ..       ••        ••  (&2.       @        "           ••                 "             "       ••       "  <3&2 

• 

o 

< 

c 

@ 

e 

e 

e 

A 

loisfure  difference  at  time  of  glu/'n 
Laminated  construction  =  0.0  %  " 

7 

r 

loisfure  difference  at  time  ofgluin 
Laminated  construction  =  3.0  "70 

? 

• 

*( 

\ 

• 

| 

>       0 

® 

o 

^ 

O 

© 

T)    ' 

i 

o 

i 

?e 

*'- 

e 

© 

e 

fc 

e 

S%e 

A 

loisfure  difference  at  time  ofgluin 
Laminated  construction  =  4-.O  °7o 

? 

A 

ioisfure  difference  at  time  of  gluing 
Laminated  construct  ion  =  7.0°7« 

r 

c 

) 

i 

t 

• 

©t 

i 

e 

*    « 

o 
o 

1 

x> 

© 

< 

\ 

39 

e 

e 

A 

loisfure  difference  at  time  ofgluin 
Laminated  construction  =8.Q  % 

9 

A 

loisfure  difference  at  time  ofgluin 
Laminated  construction  =  II.O  "7, 

9 

\ 

12 


16 


20 


12  16  £0  4  8 

Moisture  content  at  test  -  Per  cent 
PLATE  12.— Results  of  tests  showing  relation  between  tensile  strength  across  grain  of  laminated  specimens  (glued  at  uniform  and  at  nonuniform 
moisture  conditions)  and  tensile  strength  across  grain  of  free  specimens  after  both  have  been  subjected  to  various  atmospheric  conditions. 


INTERNAL   STRESSES   IN    LAMINATED   CONSTRUCTION. 


23 


Unit  strength  glued  piece 

1.2 

1.0 
.8 
.6 
1.2 
1.0 

.8 
<b 

0 

.*  .6 
<X 
<u     . 
«j  .4 

{ 

$1.2 

0 

S/.0 

I* 

S, 

« 

) 

5    1.0 

\      .8 
>> 

3       6 

1     * 

> 

1      2 

1     1.0 

.8 
.6 
1.2 
1.0 
.6 
.6 
.4 
.2 

AFRICAN  MAHOGANY 

9  Tested  offer  gluing                                                9  Tes  ted  otter  gluing 
O         ••             ••       conditioning  in  room  No.  1           Q         "             "      conditioning  in  room  No.  3 

®      ..        -             -          •••     ••      »/&a     @      "        ••            ••          -.  3&2 

•        -                                            /.2&3      ®        "           "                 '•             3,2  &l 

•  • 

e 

1^° 

°0 

-> 

© 

t 

w 

•  w 

i 

© 

D 

e 

e 
@ 

e 

a 

e 

e 
e 

w 
e 

e 

e 

e 

e 

© 

® 

A 

loisture  'difference  of  time  of  gluing 
L  am/noted  construction  =  0.0°7o 

> 

A 

loisture  difference  off/me  ofg/uim 
Laminated  construction  *  3.O  °70  . 

7 

•  • 

e 
-c®, 

n  °( 

• 

•    & 

& 

9 

1 

> 

• 

©  o 
•n 

O 

^ 

, 

e 

e 

9 

eu 

i 

*       tP 

e  e 

® 

® 

» 

e 

e 

e 
e 

; 

e 

/ 

1oist 
Loi 

ure  difference  off/me  of  g/uin 
-ninated  construction  =4.0°Io 

? 

Moisture  difference  of  time  of  gluint. 
Laminated  construction  -  7.O°70 

7 

• 

I 

• 

€ 

§« 

o 

o 

•• 

<* 

e 

o 

O 

ee 

ree 

e 

A 

e 

»  e 

e 

9 
ffi 

^ 

A 

loisture  difference  at  time  of  gluin 
L  ominofed  construction  =  8.0  °70 

? 

/ 

•foisfure  difference  of  time  of  g/uin 
Laminated  con  str  u  cf  io  n^  -II-O  % 

9 

4               a               12              16              20              4               <3                12              16              20 

Moisture  content  of  test  -  Per  cent 
PLATE  13.— Results  of  tests  showing  re'.ation  between  tensile  strength  across  grain  of  laminated  specimens  (glued  at  uniform  and  at  nonuni.'orm 
moisture  conditions)  and  tensile  strength  across  grain  of  free  specimens  after  both  have  been  subjected  to  various  atmospheric  conditions. 

86540—22 4 


24 


REPORT   NATIONAL  ADVISORY   COMMITTEE   FOR   AERONAUTICS. 


A  4.u  *  -~  Unit  strength  glued  piece 

1.2 
1.0 
.8 
.6 

i.a 

1.0 

,8 

\ 

.*  .6 

K 

^ 

2.e 
*•* 

1.2 

> 

i  '•" 

:     .8 

h 

t      .6 

I      -4 
>> 
3 

5     •* 

*  /.* 

.5 

.6" 

/£ 
/.o 

•8 
.6 
A 
.2 

Rrn  GUM 

9  Tested  after  gluing                                               9  Tested  after  gluing 
0       ••           ••      conditioning  in  room  No.  1         e        "           "     conditioning  in  room  No.  3 

4 

4 

i. 

-   . 

c1  "• 

o 

4 

e 

o 

e 

.    i 

\ 

e 

0 

* 

^ 

i 

'"e 

e 

• 

»  «r- 

/ 

7 

A 

to  is  Jure  difference  at  time  ofg/uini 
L  aminated  cons  true  tion  =  0.0°7o 

A 

loisture  difference  at  time  ofglu/nt 
Laminated  construction  =3.O°?o 

1 

4 

i 

PSlj 

o 

o 

® 

• 

< 

t" 

o  c 

i 

e 

c 

©^ 

1 

» 

OtO} 

e 

i 

*5e 

e 

• 

<§> 

e 

9 

A 

loisture  difference  at  time  ofgluin 
Laminated  construction  ~4.O°7o 

7 

A 

loisture  difference  at  time  ofgluin<. 
Laminated  construction-'  7.O°7o' 

r 

®   c 

• 

D 

ir 

. 

• 

W9 

e 

A« 

e 

e 

": 

( 

) 

A 

loisture  difference  at  time  of  gluim, 
L  aminated  construction  =  8.O°7o 

7 

Al 

'oisture  difference  at  time  of  gluing 
L  am  /noted  cons  truction  *  II.  fft  °7o 

4               8               12              16              20              4                8                ia              16              2O 

Moisture  content  at  test  -  Per  cent 

— Results  of  tests  showing  re  ation  between  tensile  strength  across  grain  of  laminated  specimens  (glued  at  uniform  and  at  nonuniform 
moisture  coniitions)  and  teosi'e  strength  across  grain  of  free  specimens  after  both  have  been  subjected  to  various  atmospheric  conditions. 


INTERNAL   STRESSES   IN   LAMINATED   CONSTRUCTION. 


25 


.0 

•£ 

lv 

j; 


10 

$> 

I 

N: 


i.a 

1.0 
.8 
.6 

i.a 
w 

.8 
.4 

.8 
.6 

.8 
.6 

.8 
.6 
.4 

.a 

NORTHERN  WHITE  OAK 

e  Test<ed  after  g, 
g  in  room  No.  1          e        "           "     cc 
.•  /&a     e 

e  Tested  after  gluing 
o         ••            ••      conditionir 
© 

'uing 
nditioning  in  room  No.  3 
a             M      n      »3&2. 
"      ••     »3.2&t 

• 

o® 

i 

o 

§.< 

I 

>*    e 

0° 

Q 

• 

o 

@e 

I 

e 

®m 

a      e 

e 

•    e 
© 

© 

e 

9 

a 

A 

loisture  difference  at  time  ofgluint 
Laminated  construction  =  O.OC70 

? 

A 

loisfure  difference  attime  of  gluing 
Laminated  construction  =  3.  0°7o  " 

' 

• 

l 

i 

I 

S 

.« 

> 

e 
-      ( 

i 

$ 

1 

« 

> 

O 

• 

> 

© 

y 

@ 

F" 

'% 

e 

I 

e 

© 

A 

loisture  difference  at  time  ofg/uim 
Laminated  construction  =4.O°lo 

7 

A 

loisture  diffe 
Laminated 

rence  at  time  ofgluin\ 
construction  =  7.  0  °7o  ' 

7 

• 

0 

% 

9 

o 

• 

r 

u 

c 

© 

?e 
© 

£ 

© 

e 
e 

e 

i 

A 

loisture  difference  aftime  of  gluing 
Laminated  construction  -8.O°70 

A 

loisfure  difference  at  time  of  gluint. 
Laminated  construction  =  ll.O  ^o 

7 

4            Q           ia          16          £0           4            8           ia           <e          ao 

Moisture  content  at  test  -  Per  cent 
PLATE  15.— Results  of  tests  showing  relation  between  tensile  strength  across  grain  of  laminated  specimens  (glued  at  uniform  and  at  ncnuniform 
moisture  conditions)  and  tensile  strength  across  grain  of  free  specimens  after  both  have  been  subjected  to  various  atmospheric  conditions. 


26 


REPORT   NATIONAL  ADVISORY   COMMITTEE   FOR  AERONAUTICS. 


L/ntt  strength  glued  piece 

/.a 

1.0 
.8 
6 
1.2 
/.O 
8 
^    6 
\.4 

\'8 

/.a 
> 
|    i.o 

\      .6 
5       4 

R 

£ 
r 

^     J.O 

.8 
.6 
1.2 
I.O 
.8 
.6 

4 
.3 

NORTHERN  RED  OAK 
e  Tested  after  gi 
iginroomNo.l          Q        ••            "     co 
-'     -        -I&2      e 

/  ^jO  3            /TJ 
*•                         "^  t  Of  »-*           W 

O  Tested  after  g/uing 
O       ••          ••      conditionit 
© 

'uing 
nditioning  in  room  No.  3 

% 

> 

e>> 

B 

c 

o 

• 

c 

ouo 

1 

o 

e 

%1 

A 

e 

a     i 

4 

I 

«     ' 

i 

1 

e 

A 

'oisture  difference  ottime  ofgluinc 
Laminated  construct/on  =  O.O°7o  ' 

i 

A 

loisture  difference  at  time  ofg/uinc. 
Lam/noted  construct/on  =  3.O°7o  ' 

7 

t 

o 

Q 

« 

® 

j«® 

r 

; 

3 

^ 

.* 

t 

E 

^( 

? 
jt 

e 

e 

e 

e  « 

e 
> 

© 

A 

loisture  difference  at  time  ofg/uini 
Laminated  construct/on  =  4.0  °7o  ' 

7 

A 

loisture  difference  at  time  of  g/uini 
Laminated  fOnstruction-  7.0°7o 

7 

.ffl 

< 

• 

. 

t 

> 

j 
» 

^ 

o 

>,  • 

a 

5 

% 

tf  J 

! 

• 

• 

e 

e 

® 

A 

loisture  difference  at  time  of  gluin 
Laminated  construction  *8.O°7o 

7 

A 

loisfure  difference  at  time  ofgluini 
Laminated  construction  =//.0°7a 

7 

4                 8                ia               16               £O               4                 8                 12               16               e0 

^^__^__  Moisture  content  at  test  -Percent 

PLATE  16.— Resu'ts  of  tests  showing  re'ation  between  tensile  strength  across  grain  of  laminated  specimens  (glued  at  uniform  and  at  nonuniform 
moisture  conditions)  and  tensi'.e  strength  across  grain  of  free  specimens  after  both  have  been  subjected  to  various  atmospheric  conditions. 


INTERNAL   STRESSES   IN   LAMINATED   CONSTRUCTION.  27 

That  internal  stresses  of  serious  magnitude  can  be  developed  by  gluing  together  material 
of  different  moisture  contents  is  shown  by  the  failure  of  maple  specimens  soon  after  manufacture. 
While  this  action  occurred  only  in  the  maple  specimens  for  which  the  rate  of  shrinkage  was  high, 
internal  stresses  would  likewise  be  set  up,  to  a  lesser  degree  perhaps,  in  the  other  species  with 
relatively  high  rates  of  shrinkage.  The  rapid  regain  in  strength  in  maple  specimens  which  did 
not  fail  immediately  after  gluing,  shows  that  the  internal  stresses  are  developed  to  a  maximum 
and  then  die  out,  probably  at  a  constantly  decreasing  rate.  The  results  for  other  species  con- 
form to  the  same  theory  and  indicate  that  if  specimens  are  allowed  to  condition  for  a  long  period 
of  time,  internal  stress  will  completely  disappear. 

CONCLUSIONS  FROM  SERIES  B  TESTS. 

From  this  series  of  tests  the  following  conclusions  seem  warranted: 

1.  The  gluing  together  of  laminations  of  different  moisture  contents  gives  rise  to  internal 
stresses  on  account  of  the  unequal  swelling  and  shrinkage  as  the  laminations  approach  a  com- 
mon moisture  content.     This  results  in  a  weakening  across  the  grain  of  the  structure,  which 
may  be  of  sufficient  magnitude  to  cause  rupture  of  members  of  the  laminated  structure. 

2.  When  a  laminated  structure  is  composed  of  members  whose  moisture  contents  are 
not  the  same,  the  moisture  in  the  wood  tends  to  equalize,  and  stresses  are  set  up  in  the  structure 
through  unequal  shrinkage  or  swelling  of  the  members.     These  stresses  die  out,  leaving  the 
structure  stress-free  but  with  changed  dimensions:    and,  if  the  elastic  limit  of  the  wood  has 
not  been  exceeded,   the  strength  is  not  affected.     If  the  structure  is  subjected  to  further 
moisture  change,  no  stresses  are  induced,  since  all  members  have  reached  the  same  condition 
and  act  together. 

3.  Animal  glue  used  in  these  tests  does  not  set  properly  when  the  laminations  are  of  rather 
high  moisture  content.     The  exact  point  where  unsatisfactory  results  occur  can  not  be  deter- 
mined from  the  data  secured,  but  it  appears  to  be  between  15  and  38  per  cent.     Also,  glued 
specimens  placed  under  conditions  tending  to  produce  a  moisture  content  of  from  15  to  18 
per  cent  in  the  wood  indicate  that  the  glue  softens  and  permits  the  laminations  to  be  easily 

pulled  apart. 

ANALYSIS  OF  RESULTS  OF  SERIES  C. 

THE  INFLUENCE  OF  COMBINING    HIGH-DENSITY  AND    LOW-DENSITY  MATERIAL  ON  THE  STRENGTH  OF  LAMINATED 

CONSTRUCTION. 

Glued  and  free  specimens  were  made  in  which  faces  and  core  differed  in  density.  The 
direction  of  the  annual  rings  within  a  species  and  the  moisture  content  at  gluing  were  made 
uniform. 

In  this  series  the  elastic  properties  of  the  wood  also  affect  strength  ratios  of  the  test,  high- 
density  wood  usually  having  greater  strength  7  and  elasticity  than  low-density  wood.     Results 
of  the  tests  are  shown  in  plates  16  to  21,  inclusive. 
CENTRAL  AMERICAN  MAHOGANY. 

This  is  a  species  of  relatively  low  radial  tensile  strength  across  the  grain.  Small  variations 
in  strength  result,  therefore,  in  considerable  variation  of  strength  ratios,  as  seen  in  plate  16, 
where  laminated  specimens  of  uniform  density  gave  variable  results  with  or  without  moisture 
changes.  Theoretically,  these  specimens  developed  no  internal  stress  with  moisture  changes, 
and  the  strength  ratios  should  equal  unity. 

In  the  speciinens  made  of  mixed-density  material,  greater  variations  were  obtained, 
possibly  due  to  variation  in  elastic  properties,  to  the  presence  of  internal  stresses,  or  to  both. 
Internal  stresses  should  come,  however,  only  with  changes  in  moisture  content,  because  the 
laminations  were  brought  to  equilibrium  before  being  glued.  Specimens  in  which  moisture 
changes  had  taken  place  showed  only  slight  reduction  in  strength  ratio  below  those  having  no 
change  in  moisture  content,  indicating  that  if  internal  stresses  were  present,  they  were  of  small 
magnitude. 

*  Bulletin  No.  676,  U.  S.  Department  of  Agriculture,  "The  Relation  of  the  Shrinkage  and  Strength  Properties  of  Wood  to  its  Specific  Gravity," 
by  J.  A.  Newlin  and  T^R.  C.  Wilson. 

86540—22 5 


28  REPORT   NATIONAL  ADVISORY   COMMITTEE   FOR   AERONAUTICS. 

HARD  MAPLE. 

This  is  a  species  of  relatively  high  radial  tensile  strength  across  the  grain,  and  strength 
ratios  (pi.  17)  for  specimens  of  uniform  density  were  less  variable  than  for  Central  American 
mahogany.  Although  greater  variation  of  strength  ratios,  due  probably  to  variation  in  elastic 
properties,  is  found  in  specimens  of  mixed  density,  average  ratios  remain  the  same  for  all 
moisture  changes,  indicating  that  only  slight,  if  any,  internal  stresses  were  present  at  time 
of  test. 
PHILIPPINE  MAHOGANY-. 

This  is  a  species  of  relatively  low  radial  tensile  strength  across  the  grain,  and  strength 
ratios  for  this  species  (pi.  18)  were  somewhat  variable  for  specimens  of  uniform  density. 
Specimens  of  mixed  density  gave  strength  ratios  somewhat  more  variable,  but  with  the  same 
average  over  all  ranges  of  moisture  change,  indicating  but  slight,  if  any,"  internal  stresses 
present  at  the  time  of  testing. 
YELLOW  POPLAR. 

This  is  a  species  of  medium  but  variable  radial  tensile  strength  across  the  grain,  giving 
quite  variable  strength  ratios,  even  for  specimens  of  uniform  density  material  "(pi-  21).  Speci- 
mens of  mixed  density  material  showed  a  similar  variation  in  the  tensile  strength  across  grain, 
but  indicated  at  the  time  of  test  no  development  of  appreciable  internal  stresses  due  to  change 
in  moisture  content. 
YELLOW  BIRCH. 

This  is  a  species  of  relatively  high  radial  tensile  strength  across  the  grain,  and  strength 
ratios  for  uniform  density  specimens  are  only  moderately  variable  (pi.  19).  Specimens  of 
mixed  density  gave  no  indication  of  internal  stress  development  with  moisture  changes,  except 

those  having  greatest  density  difference  (  jy  =  .779—  .761  j.     The  values  here  are  so  few,  how- 
ever, that  they  can  not  be  taken  to  indicate  serious  stress  conditions  inconsistent  with  values 
for  specimens  of  other  densities. 
AFRICAN  MAHOGANY. 

This  species  is  of  comparatively  low  tangential  tensile  strength,  and  quite  variable  strength 
ratios  were  obtained  for  specimens  of  uniform  and  of  nonuniform  density  material.     The  results 
(pi.  18)  do  not  indicate  moisture  changes  causing  any  serious  development  of  internal  stress  at 
time  of  test. 
NORTHERN  WHITE  OAK. 

This  species  is  of  comparatively  high  tangential  tensile  strength  across  the  grain,  giving 
uniform  strength  ratios  (pi.  20).     Specimens  of  mixed  density  gave  strength  ratios  as  great  as 
those  of  uniform  density,  indicating  that  there  were  no  internal  stresses  at  time  of  test. 
NORTHERN  RED  OAK. 

This  species  also  possesses  relatively  high  tangential  tensile  strength  across  the  grain, 
and  the  strength  ratios  (pi.  20)  are  comparable  in  uniformity  with  those  of  other  species.     No 
indications  of  internal  stress  at  time  of  test  were  found. 
RED  GUM. 

This  species  is  lower  in  tangential  tensile  strength  across  grain  than  the  oaks  and  is  also 
more  variable.  Results  (pi.  19)  for  specimens  of  mixed  density  do  not  indicate  lower  strengths 
than  for  those  of  uniform  density,  nor  were  internal  stresses  apparent  at  time  of  test. 

CONCLUSIONS  FROM  SERIES  C  TESTS. 

The  results  of  these  tests  do  not  indicate  that  internal  stresses  of  any  serious  magnitude 
are  developed  by  the  unequal  shrinkage  properties  of  material  of  rather  extreme  densities 
within  the  species  studied.  While  stresses  may  be  set  up  in  laminated  construction  containing 
material  of  the  various  densities  found  within  a  species,  they  are  apparently  of  small  magni- 
tude and  within  a  comparatively  short  time  become  so  small  that  they  fall  within  the  variation 
of  the  strength  properties. 


INTERNAL   STRKKSKS   IN   LAMINATED   CONSTRUCTION. 


29 


Ave.fJnlt  strength  glued  piece 

1.2 
I.O 
.8 
.6 
1.2 
1.0 
.8 
.6 
1.2 
I.O 

8* 

.0) 

|.6 
1 
** 

^1.0 

^ 

"*       r, 

V)  .8 

•K 

|.6 

If 

^/4 
i 

>     /.2 
5 

:   i.o 

\     .8 
i 

S     -6 

L) 

3*    t-2 
i 

C     '-° 
.8 

.6 
1.2 
f.O 
.8 
.6 
1.2 
/.O 
.8 

R 

HARD  MAPLE 

g 

c 

0 

ocp 

o 

O 

los 

^° 

R 

0 

1  1, 

lifot 

-m 

h/gt 

i  de 

nsi 

tv  ' 

oc^ 

30    0 

O 

c 

fcU 

TJ 

e 

e 

e 

e 

e 

e 

e 
e  «  i 

9« 

Q 

e  e 

e 
^ 

»     « 

« 

% 

1 

/n/fc 

irm 

lov 

/  de 

>nsi 

fv 

« 

°f 

e 

s  oS< 

£ 

-        6 

e 

0 

'J 

•• 

• 

• 

• 

a 

• 

1 

• 

•*oS 

I 

0 

c 

)OI 

-  .8 

en 

» 

• 

0 

• 
e 

& 

*+  < 

k 

5 

D' 

oc/ 

® 

1 

© 

^ 

© 

I 
o 

D 

t 

'59 

.    A 

00 

© 

® 

• 

§ 

I 
i 

D' 

-  ,c 

.a 

D  =  Density  of  /ow  density  member. 

i                             i          i 

O  '=  Density  of  high  density  member. 

iii                 i                 iii 

CENTRAL  AMERICAN  MAHOGANY 

o 

c 

D 

?-, 

o 

30°0 

°<5f 

o 

1  o 

0° 

R° 

O 

O 

o 

C 

1  Ir 

?/XOA 

-m  i 

high 

'  de 

nsh 

'»/ 

< 

u 
>      G 

°0 

°0 
0  0 

%d 

) 

n 

ptbb 

DO 

! 

i 

y 

O 

o 

c 

'     0 

{ 

-°^_ 

W 

9 

a 

e  o 

ee' 

jiV 

»  oc 

e 

8 

e 

9 

1  It 

7/^0 

rm 

low 

1  de 

nsh 

9 

O 

0 
9      8 

o 

i8 

e    c 

~V*~ 

e 

A 

c« 

» 

e 

( 

• 

e 

• 

• 

A 

o 

• 
• 

& 

Z7 

0 

00 

.     7 

^n  - 

I 

•    < 

• 

i 

i 

i 

Z7' 

—  .0 

DC/ 

• 

• 

• 

<B 

9 

• 
ffl 

0 

-j 

43 

-.7i 

nn 

—  • 

® 
i 

% 

® 

• 

0' 

JU 

\  ^ 

12  8  4  0  4  8 

Loss  Moisture  change -Per  cent  Gain 

PLATE  17.— Results  of  tests  showing  relation  between  tensile  strength  across  grain  of  laminated  specimens  (glued  of  uniform  density  and  of  non- 
uniform  density  stock)  and  tensi'.e  strength  across  grain  of  free  specimens  after  both  have  been  subjected  to  various  atmospheric  conditions. 


30 


REPOET   NATIONAL  ADVISORY   COMMITTEE   FOR   AERONAUTICS. 


Ch 


5 

J 

<0 
Q)' 

0 
^ 

\: 


L2 
1,0 
.8 
6 
1.2 
1.0 
.6 
.6 
1.2 
10 
.6 

W 
.8 

1.2 

.6 

.6 

1.0 
.8 
6 
1.2 
1.0 
.8 
.6 
12 
1.0 
6 

£ 

PHILIPPINE  MAHOGANY 

r\ 

; 

L 

o 
p  f 

??<» 

J     R 

0 

T° 

o 

///• 

i/for 

~m 

>  dt 

ns/'i 

O 

o 

o 

do  • 

O 

o 

§ 

e 

0 

O 

o 

o 

0 

0 

e 

e 

o 

/ 

Inifc 
i 

rm 

/OV 

/  dt 

rw; 

^L/ 

i 

! 

w    < 

8 

i 

9 

*'\ 

^ 

e 

9 

Q   9 

^ 

^ 

i 

y 

e 

i 

to 

e 

• 

/ 

7 

.85 

0  - 

<9(9( 

* 

• 

1. 

•  | 

: 

•* 

• 

* 

« 

e 

i 

?' 

9 

• 

©  ' 

, 

C 

i 

Z? 

-) 

•  qq 
<j<j 

_    7 

t:n^ 

Q 

0       <T 

*' 

•> 

©e  1 

>     ^ 

> 

1 

Z7' 

' 

ou^ 

D* 

Density 
l 

of     /ow  density  r 

OC/T 

)A>er-, 

• 

a 

D'- 

=  Density  of  high 

density 

ArRICAN  MAHOGANY 

Q 

o 

< 

'     X) 

O 
O 

oo  6 

P 

9 

i 

i 

//r 

/for 

~m  i 

de 

ns/J 

•i/ 

?     * 

O 

>       0 

u  o 

o*-> 
ex  o 

ex 

8 

0* 

>0 

c 

> 

c 

9 

i 

9 

/ 

e 

c 

®  t 

3 

? 

« 

i 

1 

>  *  c 

0 

/ 

Inifc 

T/D 

lo\Ai 

d<° 

ns/1 

9 

W         Q 

o 

9 

s 

e  w 

>     0 

e 

o 

1 

( 

1 
I 

0 

9 

| 

• 

* 

, 

>  • 

* 

• 

D 

92 

-.8 

/"»/"> 

• 

• 

9     < 

* 

J    • 

• 

• 

• 

D' 

-  .c 

UU 

* 

| 

C 

i 

( 

0 

© 

C 

1 

D 
D1 

7 

S3- 

7 

0/0 

© 

0 

C 

> 

1 

D 

0     t 

f 

—  .  / 

.  /( 

yC/ 

1 

12  Q  4  048/2 

Loss  Moisture  change -Per  cenf  Gain 

PLATE  18. — Results  of  tests  showing  re'.ation  between  tensile  strength  across  grain  of  laminated  specimens  (glued  of  uniform  density  and  of  non- 
uniform  density  stoik)  and  tensile  strength  across  grain  of  free  specimens  after  both  have  been  subjected  to  various  atmospheric  conditions. 


INTERNAL   STRESSES   IN   LAMINATED   CONSTRUCTION. 


31 


A  ^  ^  ^  4U  ^  .  Unit  strength  g/ued  piece 

1.4 

/.a 

1.0 
.8 
.6 

1.0 
.8 

/.a 

1.0 
.8 

I   .6 

^ 

1" 

^   .8 

,6 
> 
) 

*          /  P 

r> 

i    /.o 

>       6" 
) 

\ 

1.0 
& 
.6 

/.a 

YELLOW  BIRCH 

o 

Cb 

c 

oC 

°0 

u 

°    i°o 

_^Tn    9 

9  £ 

o 

o 

D 

fir 

v'/b/ 

~m 

de 

ns'n 

4-.i 

o 

°o 

o   5 

n 

o 

o 

n 

y- 

o 

t 

- 

•   8 

? 

o 
e 

e« 

ft 

* 

\ 

e 

1  li 

lifoi 

~m 

(ow 

del 

7S/t. 

e 

1 
e 

*/ 

o 

1 
1 

5 

KJ 

e 

1 

"9 

e 

y 

* 

4 

- 

D 

=  .0 

^6 

-  .8 

c  r\ 

• 

. 

• 

D1 

5U 

* 

• 

®« 

1 

® 

®@ 

®    . 

® 

D 

149 

-  .6 

f>/~> 

I 

/ 

® 

5P" 

• 

: 

© 

\ 

D1 

-  .c 

uu 

1 

e 

e 

£ 

D 

'79 

-.7 

r*  i 

0 
e 

e 

D1 

bl 

D  =  Density  of  low  density  memb 

J                                         i    . 

e/". 

Z/  =  Density  of  high  density  mem  bt 

?r. 

RED  GUM 

o 

o 

Q) 

o 

0               ° 

0  p 
0   0 

o 

^°ft 

n 

^ 

o 

) 

o 

n 

///- 

lifor 

•m  , 

hiqh 

'  de 

nsil 

y- 

<b 

o 

o 
oo 

°8 

°0 

0 

>6 

ou 

0 

» 

t 

< 

U 
a 

*  ^i 

r« 

a 

-e 

*ls 

e 

/// 

-lifot 

-m 

low 

der 

isit^ 

** 

e 

^ 
«* 

. 

» 

e  • 
» 

P 

V 

9 

. 

D 

^54 

-   ft 

nn 

i.o 

. 

i 

• 

• 

* 

; 

• 

D' 

.0 

'JU 

1.0 
.8 
K 

• 

® 

® 

\ 

® 

D 

'49 

.    c 

cc 

& 

® 

1 

qao 

®  m 

^ 

® 

D' 

-  .  / 

.0 

Do 

1 

8  4  04  8/3 

Loss  Moisture  chonge  -Percent  Goin 

PLATE  19. — Results  of  tests  showing  relation  between  tensile  strength  across  grain  of  laminated  specimens  (glued  of  uniform  density  and  of  non- 
uniform  density  stock)  and  tensile  strength  across  grain  of  free  specimens  after  both  have  been  subjected  to  various  atmospheric  conditions. 


32 


REPORT   NATIONAL  ADVISORY   COMMITTEE   FOR   AERONAUTICS. 


1.4 
I.B 
1.0 
.8 
i.S 
1.0 
.8 
1.2 
1.0 
JB 

%•'•* 
.*) 

§*/.£> 

1; 

£.8 
$ 

r 
%  ^ 

^  a 
S  •* 

Jf.<? 
^ 

j    i.a 

3 

:   /c? 

!>    .8 
i 

;      & 
\ 

[•   i.o 

!  •* 

/£ 
/0 
.£ 

/2 
X0 
.8 

/.a 
w 

.8 
.s 

NORTHERN  WHITE  OAK 

O 

I 

b 

C 
p 

0 

80 
i 

CD 
o 

C 

o 

//• 

i/fOf 

~/T7 

high 

i  de 

nsi\ 

*v 

0 

0      C 

d   o 
o 

9S 

r 

£° 

CD 

o 

0 

3 
3 

y 

o 
e^ 

»W 

e 

// 

nifo 

"ID 

low 

det 

75-/T, 

a 
o 

*e 
n«^ 

^      " 

8e» 

y 

'   s 

%w 

• 
.  « 

D 

9O 

-  .8 

nn 

4 

i 

1 

• 

• 

• 

D1 

uu 

_^u  ^  .  Ave.,Unit  strength  glued  piece 

c 

© 

m 

© 

©   ^ 

1 

D 

>79 

-  .7 

© 

4 

€ 

© 

I 

<; 

1 

© 

I 

D' 

• 

ou 

V 

© 

1 

e 

e 

1 

e 
e  e 

e 

-e&- 

e 

D 

'49 

c 

80- 

e 

e 

Q 

j 

3e 

D1 

.0 

D  ~  Density 

of  low  density  ITU 

'y 
?mber. 

q*a 

?ns/ty  of  high  de 

ISf'^ 

'  me. 

mbt 

?/-. 

NORTHERN  RED  OAK 

ft 

°1° 
7      °f° 

r 

Olo 

9^ 

Q> 

m 

o 

o 

l/r 

i/'for 

~m  i 

h/gr 

de 

nsh 

o 

'     O 
0 

o  o) 

r° 

°1 

0 

O 

o 

0 

o 

1 

8 

e 
*  « 

e« 

fiB 

e 

* 

4 

®    0 

0 
=^ 

c 

II, 

nifo 

r/77 

low 

der 

7>5/7j 

i 

o 

<*» 

ff 

"«. 

O 

I 

• 

/ 

f 

D 

=  .<? 

*3^ 

-  .8 

nn 

•  • 

• 

• 

• 

• 

D' 

UU 

e 

• 

® 

© 

® 
© 

© 

Sf    ffl 

@   • 

4 

D 

• 

'73 

-  .1 

^n 

• 

w 

©  e 

w 

© 

D' 

-  .  / 

DU 

d 

I 

e 
e 

c 

%l 

A 

D 

^ 

-  .6 

CO 

e 

D1 

-  ./ 

jj 

e  *  o  4  8  /a 

Loss  Moisture  change  -  Per  cent  Gain 

PLATE  20. — Results  of  tests  showing  re.atioii  between  tensi.e  strength  across  grain  of  laminated  specimens  (glued  of  uniform  density  and  of  non- 
uniform  density  stock)  and  tensile  strength  across  grain  of  free  specimens  after  both  have  been  subjected  to  various  atmospheric  conditions. 


INTERNAL   STRESSES   IN    LAMINATED   CONSTRUCTION. 


33 


From  this  series  of  tests  the  following  conclusion  appears  warranted: 
When  laminations  of  very  high  and  very  low  densities  are  combined  to  form  a  laminated 
structure,  change  of  moisture  content  induces  stresses  on  account  of  the  unequal  shrinkage  or 
swelling  of  the  members.  These  stresses  disappear,  and,  if  the  elastic  limit  of  the  wood  has 
not  been  exceeded,  only  a  change  in  dimension  results.  Further  changes  in  moisture  content 
induce  new  stresses.  Within  a  single  species  the  stresses  so  induced  are  relatively  small,  how- 
ever, and  not  likely  to  be  serious  except  in  extreme  cases. 


QJ 

St 

1 

QJ 

i 
* 

? 

1 

*> 

•<. 

§ 

i 
< 

c 
1 

4 

( 

t. 

«r 

1.2 

HJ 
U  1.0 

•? 

1  £ 

^  /0 

i 
/0 

"     .<9 
> 

.6 

*   i.a 

1.0 
.8 

YELLOW  POPLAR 

o 

0 

c 

o 

Q 

£ 

.1° 

1      ^ 

o 

o 

Q 

• 

o 

O 

O 

** 

P 

o 

o 

c 

0 

o 

Uniform  high  density 

o 

o 

o 

e 

o 

e 
« 

e 
i 

e 

e     a 

e 

Uniform  low  density- 

\ 

6 

o 

5 

§5 

o 
l 

f«* 

o 

£ 

e 

~r 

» 

se 

e 

i 

^ 

?55 

o 

: 

*. 

* 

*  • 

•  •' 

• 

o1 

oo 

* 

* 

- 

(9 

• 

. 

6 

D 

79 

~.6> 

9 

. 

. 

• 

D' 

-  .7 

07- 

.6 

D  -  Dens/  ty  of  tow  de  nsi+y     < 

O  =  Density  of 

hi'gh  dei 

->sil) 

'  me 

mbe. 

T. 

12               8               4                 048/2 

Mo  is  ture  change  -Per  cen  /  Coin 

PLATE  21. — Results  of  tests  showing  relation  between  tensile  strength  across  grain  of  laminated  specimens  (glued  of  uniform  density  and  of  non- 
uniform  density  stock)  and  tensile  strength  across  grain  of  free  specimens  after  both  have  been  subjected  to  various  atmospheric  conditions. 

DISCUSSION  OF  RESULTS. 

The  outstanding  feature  of  this  series  of  investigations  is  the  decrease  in  magnitude  of 
internal  stresses  with  time.  Although  shrinkage  governs  the  development  of  internal  stresses, 
the  time  factor  affects  the  permanency  of  these  stresses.  In  laminated  construction  containing 
plain-sawed  and  quarter-sawed  material,  results  showed  internal  stresses  in  specimens  tested 
after  a  comparatively  short  period  of  conditioning,  but  showed  absence  of  such  stresses  where 
specimens  were  conditioned  for  comparatively  long  periods  before  test,  due  consideration  being 
given  to  the  rate  of  moisture  change  peculiar  to  the  species.  Thus  there  is  strong  indication  that 
internal  stresses  die  out  under  constant  uniform  atmospheric  conditions. 

Evidence  from  the  series  in  which  material  of  different  moisture  contents  was  glued  together 
showed  development  of  internal  stresses  in  some  specimens  of  maple  sufficient  to  cause  rupture. 
In  other  specimens  similarily  made,  which  did  not  fail  under  internal  stresses  and  which  were 
allowed  to  condition  under  uniformly  constant  atmospheric  conditions,  there  was  evidence  of  a 
remarkable  regain  in  strength,  corroborating  further  the  theory  that  internal  stresses  die  out 
in  time,  provided  atmospheric  conditions  remain  constant.  Results  from  the  other  species  of 


34  REPORT   NATIONAL  ADVISORY   COMMITTEE   FOR  AERONAUTICS. 

this  series  showed  the  development  of  internal  stresses  to  a  lesser  extent,  and  likewise  showed 
regain  in  strength  with  continued  conditioning. 

The  series  in  which  variable  shrinkage  due  to  density  difference  was  studied  indicated  that 
stresses  developed  from  this  source  are  much  less  significant  than  those  caused  by  moisture 
differences  at  time  of  gluing,  or  by  the  combining  of  plain-sawed  and  quarter-sawed  material. 
The  results  have  shown  that  internal  stress  in  numerous  species  disappears  under  constant 
atmospheric  conditions,  but  the  specimens  suffered  permanent  deformation.  This  must  be  due 
to  a  property  of  the  wood  fiber,  by  virtue  of  which  it  may  be  deformed  and  develop  resistant 
stress,  but  in  which  the  stress  gradually  disappears,  leaving  the  deformation  permanent.  Such 
property  must  be  inherent  in  the  wood  itself,  irrespective  of  the  source  of  internal  stress,  and  the 
theory  explains  the  dying  out  of  internal  stresses  in  the  laminated  specimens  of  these  tests.  In 
order,  however,  that  the  strength  of  the  wood  shall  not  be  permanently  reduced,  internal  stresses 
must  not  have  exceeded  its  elastic  limit. 

The  development  of  internal  stresses  is  due  to  unequal  shrinking  and  swelling,  and  the 
magnitude  of  stress  developed  will  vary  with  the  magnitude  of  such  inequality.  The  inequality 
of  shrinkage  within  a  species  between  wood  of  low  density  and  wood  of  high  density,  for  any  of 
the  species  studied  in  this  test,  does  not  seem  to  be  enough  to  cause  serious  internal  stress  with 
moisture  changes  of  even  10  or  12  per  cent.  Stresses  so  developed  eventually  die  out  when 
a  uniform  moisture  content  is  maintained.  Any  change  in  moisture  content  develops  new 
stresses,  which  also  eventually  disappear  under  constant  moisture  conditions. 

Between  plain-sawed  and  quarter-sawed  material  the  inequality  of  shrinkage  is  greater; 
and  larger  stresses  are  developed  with  moisture  changes.  Moisture  differences  between  lamina- 
tions at  gluing  can  develop  stresses  of  even  greater  magnitude,  capable  sometimes,  as  shown  in 
the  test,  of  causing  failure  without  application  of  external  loading. 

The  shrinkage  properties  given  in  Table  1  give  some  indication  of  the  factor  which  is  likely 
to  develop  the  greatest  internal  stress  in  laminated  construction  of  any  species.  Values  from 
Table  1  may  be  used  in  comparing  the  magnitude  of  unequal  shrinkage  in  laminated  construc- 
tion of  plain  and  quartered  material  when  undergoing  moisture  changes  after  manufacture, 
with  the  unequal  shrinkage  caused  by  gluing  together  material  of  different  moisture  contents. 

Internal  stresses  that  have  once  died  out  do  not  always  recur  with  a  change  in  moisture 
content.  When  only  moisture  differences  exist  at  the  time  of  gluing,  the  source  of  stress  dis- 
appears when  a  common  moisture  content  is  reached.  Thereafter  all  members  will  change 
moisture  content  at  the  same  rate,  and  shrinking  or  swelling  will  be  approximately  equal. 

Unequal  shrinkage  due  to  density  difference,  or  to  method  of  sawing,  does  not  perma- 
nently disappear  with  conditioning,  and  each  moisture  change  sets  up  new  stresses,  irrespective 
of  previous  moisture  contents  or  conditioning. 

The  results  of  this  test  do  not  indicate  the  rate  of  development  or  disappearance,  nor  the 
magnitude  to  which  internal  stresses  are  developed.  Failure  of  specimens  in  maple  indicated 
that  internal  stresses  beyond  the  strength  of  the  wood  may  be  developed.  Stresses  measured 
at  test  give  merely  the  stress  at  that  particular  time  and  can  not  be  taken  as  the  maximum. 
The  rate  of  development  and  disappearance  of  stress  no  doubt  varies  with  the  size  of  construc- 
tion, species  of  wood  used,  and  magnitude  of  source  of  stress,  and  can  be  determined  only  by 
an  actual  test  with  respect  to  time. 

In  commercial  practice,  the  sources  of  stress  frequently  occur  in  combination;  and  each 
lends  its  influence  with  respect  to  the  development  of  internal  stresses.  Plain-sawed  and 
quarter-sawed  material  of  different  densities  and  at  different  moisture  contents  are  frequently 
combined.  Since  gluing  at  different  moisture  contents  causes  the  greatest  development  of 
internal  stresses,  elimination  of  this  source  of  stress  is  highly  desirable.  This  can  be  accom- 
plished only  by  bringing  the  moisture  content  of  material  to  the  same  uniform  condition  before 
the  structure  is  glued  up.  Combining  plain-sawed  and  quarter-sawed  material  in  the  same 
structure  develops  stress  of  somewhat  lesser  magnitude  with  moiskire  changes.  Where  maxi- 
mum strength  across  grain,  in  the  direction  of  the  glue  joints,  is  desired  in  built-up  construction, 


INTERNAL   STRESSES   IN   LAMINATED   CONSTRUCTION.  35 

mixing  plain  and  quartered  material  should  be  prohibited,  as  it  results  in  a  weakening  of  the 
structure  which  may  be  temporary  but  will  return  with  each  subsequent  change  in  moisture 
content. 

Controlling  variables  to  eliminate  the  development  of  internal  stresses  increases  the  diffi- 
culties of  manufacture.  The  density  of  wood  is  difficult  to  determine  except  by  actual  tests, 
and  the  slight  development  of  stresses  from  this  source  can  be  more  easily  offset  by  the  use  of 
somewhat  lower  working  stresses.  Moisture  differences  before  gluing  can  be  eliminated  by 
proper  conditioning  of  the  material,  which,  although  not  inexpensive,  is  highly  desirable  because 
of  the  stresses  thereby  avoided.  Matching  material  for  uniformity  in  direction  of  annual  growth 
rings  reduces  the  amount  of  available  material  and  increases  the  cost  of  the  finished  article 
quite  appreciably.  Only  two  courses  of  action  can  be  followed  to  eliminate  the  development 
of  serious  stresses  from  this  cause.  Either  the  material  must  be  selected  to  give  uniform  match- 
ing of  grain — and  this  only  serves  to  minimize  the  development  of  stresses  with  moisture 
changes — or  the  moisture  content  of  the  construction  containing  both  plain-sawed  and  quarter- 
sawed  material  must  be  prevented  from  changing,  an  extremely  difficult  task  to  accomplish. 

The  effect  of  internal  stresses  in  airplane  propellers  can  be  minimized  by  the  proper  con- 
trol of  manufacturing.  Tests  on  airplane  propellers  have  shown  that  changes  in  moisture  con- 
tent cause  finished  propellers  to  warp  and  become  unfit  for  service.8  Preventing  such  changes 
by  maintaining  constant  moisture  content  would  also  eliminate  any  development  of  internal 
stress ;  provided  moisture  contents  at  gluing  were  uniform,  and  the  maximum  strength  of  the 
propeller  would  be  retained. 

s  "The  Influence  ~>\  Atmospheric  and  Manufacturing'Conditions  on  Airplane  Propellers."    Project  233,  dated  July  8, 1920,  by  A.  L.  Heim  and 
A.  C.  Knauss. 


APPENDIX  A.— WORKING  PLAN. 


FIG.  8.— Test  specimen. 


PURPOSE  OF  THE  INVESTIGATIONS. 

Field  observations  and  tests  of  timber  construction  involving  laminations  and  glued  joints 
have  indicated  that  differences  in  moisture  content,  differences  in  density,  the  combining  of 
quarter-sawed  with  plain-sawed  material  induce  stresses  due  to  atmospheric  conditions  that 
cause  checking  or  opening  of  the  glued  joints,  or  combine  with  working  stresses,  and  in  this  way 
contribute  to  failure. 

The  purpose  of  this  investigation  is  to  obtain  information  for  use  in  the  design  and  con- 
struction of  airplane  members  made  of  laminated  wood,  with  special  reference  to  propellers. 
Conditions  similar  to  those  of  field  service  will  be  maintained  and  controlled  and  the  test 
specimens  subjected  to  them.  Rooms  will  be  provided  in  which  there  can  be  maintained 
under  control  constant  conditions  of  temperature  and  relative  humidity.  These  conditions 

are  to  be  such  as  to  approximate  the  extreme  condi- 
tions found  in  actual  service. 

The  specific  information  sought  is : 

1.  A  comparison  of  the  strength  across  the  grain 
of  laminated  construction  made  entirely  of  quarter- 
sawed  material,  partly  of  quarter-sawed  and  partly  of 
plain-sawed,  and  entirely  of  plain-sawed  boards  under 

*~  such  conditions  as  may  take  place  after  gluing,  after 
the  seasoning  period,  or  in  transferring  the  glued  mem- 
ber from  one  condition  to  another. 

2.  A  comparison  of  the  strength  across  the  grain  of 
laminated  construction  made  of  pieces  of  different  densities,  with  the  view  of  determining  the 
limit  of  density  difference  that  may  be  safely  had  in  the  constituent  members  of  laminated 
construction  when  they  undergo  certain  atmospheric  conditions. 

3.  A  comparison  of  the  strength  across  the  grain  of  laminated  construction  made  of  pieces 
differing  in  moisture  content  at  the  time  of  gluing,  when  these  are  subsequently  allowed  to 
come  to  a  uniform  moisture  content. 

It  is  proposed  to  combine  this  information  with  data  to  be  obtained  from  service  failures 
and  data  taken  on  built-up  propellers  undergoing  the  same  conditions,  with  a  view  of  estab- 
lishing a  recommendation  as  to  the  allowable  moisture  and  density  difference  and  restrictions 
upon  the  use  of  plain-sawed,  quarter-sawed,  or  plain-sawed  combined  with  quarter-sawed 

material. 

MATERIAL. 

In  order  to  accomplish  the  purpose  of  this  project,  laminated  test  pieces  will  be  made  of 
each  of  five  species  representing  three  classes  of  wood  material  used  in  propeller  construction, 
and  tested.  Other  species  will  be  added  later,  if  deemed  advisable.  A  series  of  tests  will  be 
made  for  each  of  the  following  species  of  woods : 

Central  American  mahogany  (Swetenia  mahogani). 

African  mahogany  (Khaya  senegalensis) . 

Northern  white  oak  (Quercus  sp.). 

Northern  red  oak  (Quercus  sp.) . 

Yellow  birch  (Betula  sp.). 

Red  gum  (Liquidambar  styraciflua). 

Yellow  poplar  (Liriodendron  tulipifera) . 

Hard  maple  (Acer  saccharum). 

Philippine  mahogany  (Skorea  sp.) . 
36 


INTERNAL   STRESSES   IN   LAMINATED   CONSTRUCTION. 


37 


All  pieces  used  in  the  test  specimens  are  to  be  cut  from  clear  material  free  from  checks. 
This  material  will  be  selected  from  kiln-dried  stock  on  hand  at  this  laboratory,  for  which  com- 
plete data  on  other  wood  properties  is  available. 

TEST  SPECIMENS. 

The  test  specimens  shall  have  the  dimensions  shown  in  Figure  8.  Each  test  piece  is  to  be 
made  of  three  laminations,  a  center,  %  inch  thick,  and  sides  %  inch  thick.  The  laminations 
are  to  be  glued  together  when  practicable  in  20-inch  lengths,  making  blocks  from  each  of  which 
four  standard  specimens  for  tension  across  the  grain  are  to  be  cut.  Other  test  specimens  are 
to  be  made  up  in  the  same  manner,  but  not  glued  together.  Each  free  specimen  is  to  be  matched 
to  a  glued-up  specimen  and  serve  as  a  standard  of  comparison  for  the  glued-up  blocks.  These 
are  to  be  tested  in  the  usual  manner.  Laminations  for  the  glued  and  free  test  pieces  are  to  be 
matched  end  to  end  and  taken  as  near  each  other  as  possible.  Sketches  will  be  made  showing 
direction  of  the  annual  growth  rings  in  each  lamination  of  each  specimen. 

Sample  data  sheet. 


Stick  No. 

Wet 

weight 
stick. 

Wet 
weight 
disk. 

Dry 

weieht 
disk. 

Dry 

volume 
disk. 

% 
moisture 
content. 

Density. 

High,  av., 
or  low 
density. 

Series 
assign- 
ment. 

Paired 
with 
stick  No. 

%  M.  C. 

at 
gluing. 

Weight 
at 
gluing. 

Group 
assign- 
ment. 

1.. 

746 

17.92 

15.99 

32  1 

12  2 

0  499 

A 

I 

179 

7  0 

712 

IF    (1  10) 

2  

754 

17.96 

16.25 

32.0 

10  5 

508 

A 

I 

180 

7  0 

730 

I  F  (11  20) 

3  

752 

18.00 

16.15 

31.7 

11.5 

509 

A 

III 

5 

7.0 

722 

III  A(l  10) 

4   .    . 

742 

17.90 

15.87 

32  0 

12  8 

496 

A 

I 

206 

7  0 

703 

I  G    (1  10) 

5  

747 

17.62 

15.89 

31.6 

11.0 

.503 

A 

III 

3 

7.0 

720 

III  A(l  10) 

6 

838 

20.36 

18  46 

32  7 

10  3 

565 

H 

II 

7 

18  0 

897 

II  H  (i  10) 

7.                ... 

841 

20.34 

18.35 

32  7 

10  8 

561 

H 

II 

6 

18  0 

895 

II  H  (1  10) 

8  

721 

17.00 

15.31 

33.0 

10.8 

464 

L 

II 

17 

14  0 

742 

II  Y(51-60) 

•3  '5"- 


"•  Moisture  and  density  disc.  ^  Stick 

FIG.  9. — Sheet  showing  data  used  in  determining  physical  properties  of  material  and  assignment  to  schedule  of  working  plan. 


Red  Gum  Serleo   I                                                                     Conditioning  Record 

Glue             Room             Room             Room 
Room              No.l               No.  2               No.  J 

Teet 
Pieces 

May 

1 

2 

3 

4 

5 

6 

7 

8 

9 

10 

11 

12 

13 

14 

15 

16 

17 

18 

19 

20 

21 

22 

23 

24 

25 

26 

27 

21 

29 

30 

31 

•MWMy//A 

Group  A     1-30 

5 

T 

v/imm 

21-^0 

y 

T 

V//////////M 

41-60 

j 

T 

W////////A 

61-80 

j 

T 

y////////////////////////^ 

Group  C     1-20 

•J 

T 

'//////////////////////////I 

21-40 

-J 

T 

7////////////////////////A 

41-60 

•J 

T 

7////////////////////////A 

61-80 

J 

T 

y//w//////////////////s/tw/^ 

Group  D     1-20 

21-40 

v/////////////////////////^^^^ 

w///////////////////////^^^ 

41-60 

:^m*%^%^m<%%;m%^ 

61  -SO 

y//////////////////////////^^^ 

Group  E     1-20 

y/^/////////////////////y////////^^^ 

21-40 

y//////////////////////////^^^^ 

41-60 

'///////////////////////^^ 

61-80 

>/////////////,                     mm////, 

Group  T     1-20 

y 

I 

^%^%                             V^iW/// 

21-40 

•j 

T 

>////////////,                       w///////////, 

41-60 

j 

T 

Wmm.                               mmm 

61-80 

v' 

T 

Group  G     1-20 

'////////////,                Y///ty////////////////////, 

21-40 

'IMP////////,                   V//^/////7////////P//Ky/, 

41-60 

'/////////f/,               v////r////////////s////y///. 

61-80 

y////////////,             V/////////////////7////////. 

'/////?///////,  Y////////////////////////////^^^ 

Croup  H     1-20 

•J 

T 

21-40 

y 

T 

W/////////7/.  V//////////////////7////////////////^^ 

41-60 

•J 

T 

y////////////,  Y^//////MW/////////////M^^ 

Y//////////M  Y////////////////////////////////////////^ 

61-80 

y 

T 

•J  =    Specimens  reached  conetant  weight  in  final   conditioning  room. 
T  «            «          ware   tested. 

G.  10.  —  Chart  showing  record  of  conditioning  of  specimens.    Portion  of  chart  at  left  shows  progress  of  specimens  through  rooms  in  conditioning 
schedule. 

38 


REPORT   NATIONAL  ADVISORY   COMMITTEE   FOR   AERONAUTICS. 


MOISTURE  DETERMINATIONS. 

Moisture  determinations  will  be  made  upon  each  board  from  which  the  20-inch  laminations 
are  cut.  Three  blocks  1  inch  in  length  along  the  grain  are  to  be  cut  from  approximately  the 
third  points  of  the  board.  A  1-inch  section  is  to  be  cut  from  the  center  of  each  20-inch  block 
at  the  time  the  block  is  cut  into  test  specimens,  for  the  purpose  of  determining  the  average 
moisture  content  of  the  block.  Moisture  determinations  will  also  be  made  upon  each  test  piece 
after  rupture — one-half  of  the  broken  test  piece  is  to  be  sawed  apart  and  the  moisture  content 
obtained  for  each  lamination — the  average  moisture  content  of  the  test  piece  to  be  obtained 
from  the  other  half  en  masse.  The  test  specimens  will  be  weighed  at  such  intervals  as  are 
necessary  for  obtaining  information  on  the  rate  of  change  of  moisture  in  laminated  construction. 

MARKING. 

All  of  the  information  available  (shipment,  tree,  and  piece)  shall  be  indicated  in  the  standard 
way.  Besides  these  items  the  test  specimen  is  to  have  a  mark  giving  the  series,  the  group,  and 
the  number.  Series  are  to  be  indicated  with  roman  numerals,  groups  with  capital  letters,  and 
the  numbers  with  ordinary  Arabic  numerals.  In  such  cases,  when  test  pieces  are  made  of  the 
same  material  as  the  propellers  outlined  in  working  plan  for  project  L-233  ND  as  regards  density, 
moisture  content,  etc.,  the  test  specimen  is  to  have  a  mark  corresponding  to  the  mark  on  the 

propellers. 

SERIES  1.  -MATCHING  PLAIN  SAWED  WITH  QUARTER  SAWED. 

BIRCH,  OAK,  AND  MAHOGANY  ARE  TO  BE  TESTED. 

Preparation  oj  test  pieces. — The  center  lamination  of  each  test  piece  is  to  be  quarter  sawed 
and  the  sides  plain  sawed.  All  laminations  are  to  be  cut  from  clear  material,  free  from  checks. 
Density  shall  be  based  upon  oven-dry  weight  and  volume,  and  a  determination  shall  be  made 
upon  each  of  the  boards  from  which  the  20-inch  laminations  are  cut.  Moisture  conditions  are 
to  be  obtained  by  means  of  the  conditioning  rooms  provided  for  this  purpose.  The  average 
moisture  content  will  be  determined  by  weighing  the  test  pieces  from  time  to  time.  Each  test 
specimen  will  be  conditioned  in  consecutive  rooms,  passing  through  all  of  the  conditions  preced- 
ing that  condition  at  which  the  test  piece  is  to  be  broken.9 

The  following  temperatures  and  humidities  are  to  be  maintained  in  the  conditioning  rooms : 


Relative 
humidity. 
\ 

Tempera- 
ture. 

Glue  room    .     .         

Per  cent. 
65 

o    p 

90 

Workshop  

55 

70 

First  conditioning  room  

30 

80 

Second  conditioning  room  

60 

80 

Third  conditioning  room  

90 

80 

GROUP    A. 

Average  moisture  content  throughout  the  test  pieces  at  the  time  of  testing  to  be  that  pro- 
duced by  conditioning  in  the  glue  room.  Odd  numbers  indicate  test  specimens  that  are  glued 
up;  even  numbers  indicate  test  specimens  that  are  not  glued  up. 

Conditions  at  the  time  the  test  pieces  are  glued  together. 


Moisture  per  cent  when  glued. 

No. 

test  pieces. 

Center  quarter 

Sides  flat 

sawed. 

sawed. 

1-20 

20 

7 

7 

21-40 

20 

10 

10 

41-60 

20 

14 

14 

61-80 

20 

18 

18 

'  See  "  Purpose  of  the  Investigations,"  above. 


INTERNAL   STRESSES   IN   LAMINATED   CONSTRUCTION. 
GROUP  B. 


39 


Average  moisture  content  throughout  the  test  pieces  at  the  time  of  testing  to  be  that  produced 
by  final  conditioning  in  the  workshop  after  conditioning  in  the  glue  room.  Odd  numbers  indi- 
cate test  specimens  that  are  glued  up;  even  numbers  indicate  test  specimens  that  are  not 

glued  up. 

Conditions  at  the  time  the  test  pieces  are  glued  together. 


' 

Moisture  per  cent  when  glued. 

No. 

Number  of 
test  pieces. 

Center  quarter 

Sides  flat 

sawed. 

sawed. 

1-20 

20 

7 

7 

21-40 

20 

10 

10 

41-60 

20 

14 

14 

61-80 

20 

18 

18 

GROUP  C. 

Average  moisture  content  throughout  the  test  pieces  at  the  time  of  testing  to  be  that  produced 
by  final  conditioning  in  the  first  conditioning  room  after  conditioning  in  the  glue  room  and  then 
in  the  workshop.  Odd  numbers  indicate  the  test  specimens  that  are  glued  up;  even  numbers, 
test  specimens  that  are  not  glued  up. 

Conditions  at  the  time  the  test  pieces  are  glued  together. 


I 

Moisture  per  cent  when  glued. 

No. 

test  pieces. 

Center  quarter 
sawed. 

Sides  flat 
sawed. 

1-20 

20 

7 

7 

21-40 

20 

10 

10 

41-60 

20 

14 

14 

61-80 

20 

18 

18 

GROUP  D. 

Average  moisture  content  throughout  the  test  pieces  at  the  time  of  testing  to  be  that  produced 
by  final  conditioning  in  the  second  conditioning  room  after  conditioning  in  the  glue  room  and 
the  first  conditioning  room  consecutively.  Odd  numbers  indicate  test  specimens  that  are 
glued  up;  even  numbers  indicate  test  specimens  that  are  not  glued  up. 

Conditions  at  the  time  the  test  pieces  are  glued  together. 


Moisture  per  cent  when  glued. 

No. 

Number  of 
test  pieces. 

Center. 

Sides. 

1-20 

20 

7 

7 

21-40 

20 

10 

10 

41-60 

20 

14 

14 

61-80 

20 

18 

18 

GROUP  E. 


Average  moisture  content  throughout  the  test  pieces  at  the  time  of  testing  to  be  that  produced 
by  final  conditioning  in  the  third  conditioning  room  after  conditioning  in  the  glue  room,  work 
shop,  first  and  second  conditioning  rooms  consecutively.  Odd  numbers  indicate  test  specimens 
that  are  glued  up;  even  numbers  indicate  test  specimens  that  are  not  glued  up. 


40 


REPORT   NATIONAL  ADVISORY   COMMITTEE   FOR  AERONAUTICS. 
Conditions  at  the  time  the  test  pieces  are  glued  together.        i 


Moisture  per  cent  when  glued. 

Number  of 

test  pieces. 

Center. 

Sides. 

1 

1-20 

20 

7 

7 

21-40 

20 

10 

10 

41-60 

20 

14 

14 

61-80 

20 

18 

18 

GROUP  F. 

Average  moisture  content  throughout  the  test  pieces  at  the  time  of  testing  to  be  that  produced 
by  final  conditioning  in  the  third  conditioning  room  after  conditioning  in  the  glue  room  and  then 
in  the  work  shop.  Odd  numbers  indicate  test  specimens  that  are  glued  up;  even  numbers  indi- 
cate test  specimens  that  are  not  glued  up. 

Conditions  at  the  time  the  test  pieces  are  glued  together. 


Moisture  per  cent  when  glued. 

Number  of 

test  pieces. 

Center. 

Sides. 

1-20 

20 

7 

7 

21-40 

20 

10 

10 

41-60 

20 

14 

14 

61-80 

20 

18 

18 

GROUP  G. 

Average  moisture  content  throughout  the  test  pieces  at  the  time  of  testing  to  be  that  produced 
by  final  conditioning  in  the  second  conditioning  room  after  conditioning  hi  the  glue  room, 
work  shop  and  third  conditioning  room  consecutively.  Odd  numbers  indicate  test  specimens 
that  are  glued  up;  even  numbers  indicate  test  specimens  that  are  not  glued  up. 

Conditions  at  the  time  the  test  pieces  are  glued  together. 


Moisture  per  cent  when  glued. 

Number  of 

test  pieces. 

Center. 

Sides. 

1-20 

20 

7 

7 

21-40 

20 

10 

10 

41-60 

20 

14 

14 

61-80 

20 

18 

18 

GROUP  H. 

Average  moisture  content  throughout  the  test  pieces  at  the  time  of  testing  to  be  that  produced 
by  final  conditioning  in  the  first  conditioning  room  after  conditioning  in  the  glue  room,  work 
shop,  and  third  and  second  conditioning  rooms  consecutively. 

Conditions  at  the  time  the  test  pieces  are  glued  together. 


Moisture  per  cent  when  glued. 

XT/\ 

Number  of 

ISO. 

test  pieces. 

Center. 

Sides. 

1-20 

20 

7 

7 

21-40 

20 

10 

10 

41-60 

20 

14 

14 

61-80" 

20 

.     18 

18 

INTERNAL   STRESSES   IN   LAMINATED   CONSTRUCTION. 


41 


SERIES  2.— DENSITY  DIFFERENCE. 

BIRCH,  OAK,  AND  MAHOGANY  ARE  TO  BE  TESTED. 

Preparation  of  test  pieces. — The  laminations  for  birch  and  mahogany  are  to  be  cut  from 
plain-sawed  material  and  the  laminations  for  oak  are  to  be  cut  from  quarter-sawed  material. 
All  material  is  to  be  clear  and  without  checks.  Density  is  to  be  determined  upon  three  1-inch 
sections  cut  from  each  board  at  approximately  the  third  points.  After  the  density  has  been 
determined,  the  boards  are  to  be  selected.  Three  groups  are  to  be  made  consisting  of  boards 
having  a  comparatively  high  density,  a  comparatively  low  density,  and  a  mixed  density. 
These  test  pieces  are  to  be  marked  with  numbers  corresponding  to  numbers  on  propellers  built 
under  like  conditions  as  to  species,  moisture  content,  and  density.  Each  test  specimen  will  be 
conditioned  in  consecutive  rooms  passing  through  all  of  the  conditions  preceding  that  condition 
at  which  the  test  piece  is  to  be  broken.10 

GROUP  A. 

All  laminations  to  contain  7  per  cent  moisture  at  the  time  of  gluing  and  be  of  a  compara- 
tively high  density.  Odd  numbers  indicate  test  specimens  that  are  glued  up;  even  numbers 
indicate  test  specimens  that  are  not  glued  up. 


No. 

Number  of 
test  pieces. 

Average  moisture  content  at  time 
f  test. 

1-  20 

20 

Glue  room  conditions. 

21-  40 

20 

Work  shop. 

41-  60 

20 

First  conditioning  room. 

61-  80 

20 

Second  conditioning  room. 

81-100 

20 

Third  conditioning  room. 

GROUP  B. 


All  laminations  to  contain  10  per  cent  moisture  at  the  tune  of  gluing  and  be  of  a  compara- 
tively high  density.  Odd  numbers  indicate  test  specimens  that  are  glued  up;  even  numbers 
indicate  test  specimens  that  are  not  glued  up. 


No. 

Number  of 
test  pieces. 

Average  moisture  content  at  time 
of  test. 

1-  20 

20 

Glue  room  conditions. 

21-  40 

20 

Work  shop. 

41-  60 

20 

First  conditioning  room. 

61-  80 

20 

Second  conditioning  room. 

81-100 

20 

Third  conditioning  room. 

GROUP  C. 


All  laminations  to  contain  14  per  cent  moisture  at  the  time  of  gluing  and  be  of  a  compara- 
tively high  density.  Odd  numbers  indicate  test  specimens  that  are  glued  up;  even  numbers 
indicate  test  specimens  that  are  not  glued  up. 


No. 

Number  of 
test  pieces. 

Average  moisture  content  at  time 
of  test. 

1-  20 

20 

Glue  room  conditions. 

21-  40 

20 

Work  shop  . 

41-  60 

20 

First  conditioning  room. 

61-  80 

20 

Second  conditioning  room. 

81-100 

20 

Third  conditioning  room. 

10  See  "  Purpose  of  the  Investigations,"  above. 


42 


REPORT   NATIONAL  ADVISORY   COMMITTEE  FOR  AERONAUTICS. 

GROUP   D. 


All  laminations  to  control  18  per  cent  moisture  at  the  time  of  gluing  and  be  of  compara- 
tively high  density.  Odd  numbers  indicate  test  specimens  that  are  glued  up;  even  numbers 
indicate  test  specimens  that  are  not  glued  up. 


No. 

Number  of 
test  pieces. 

Average  moisture  content  at  time 
of  test. 

1-  20 
21-  40 
41-  60 
61-  80 
81-100 

to  to  to  to  to 

o  o  o  o  o 

Glue  room  conditions. 
Work  shop. 
First  conditioning  room. 
Second  conditioning  room. 
Third  conditioning  room. 

GROUP  E. 

All  laminations  to  contain  7  per  cent  moisture  at  the  time  of  gluing  and  be  of  a  compara- 
tively high  density.  Odd  numbers  indicate  test  specimens  that  are  glued  up;  even  numbers 
indicate  test  specimens  that  are  not  glued  up. 


No. 

Number  of 
test  pieces. 

Average  moisture  content  at  time 
of  test. 

1-20 
21-10 
41-60 

20 
20 
20 

Third  conditioning  room. 
Second  conditioning  room. 
First  conditioning  room. 

GROUP  F. 


All  laminations  to  contain  10  per  cent  moisture  at  the  time  of  gluing  and  be  of  a  compara- 
tively high  density.  Odd  numbers  indicate  test  specimens  that  are  glued  up;  even  numbers 
indicate  test  specimens  that  are  not  glued  up. 


Wn 

Number  of 

Average  moisture  content  at  time 

test  pieces. 

of  test. 

1-20 

20 

Third  conditioning  room. 

21-10 

20 

Second  conditioning  room. 

41-60 

20 

First  conditioning  room. 

GROUP  G. 

All  laminations  to  contain  14  per  cent  moisture  at  the  time  of  gluing  and  be  of  a  compara- 
tively high  density.  Odd  numbers  indicate  test  specimens  that  are  glued  up ;  even  numbers 
indicate  test  specimens  that  are  not  glued  up. 


t$f\ 

Number  of 

Average  moisture  content  at  time 

test  pieces. 

of  test. 

1-20 
21-40 

20 
20 

Third  conditioning  room. 
Second  conditioning  room. 

41-60 

20 

First  conditioning  room. 

GROUP  H. 

All  laminations  to  contain  18  per  cent  moisture  at  the  time  of  gluing  and  be  of  a  com- 
paratively high  density.  Odd  numbers  indicate  test  specimens  that  are  glued  up;  even 
numbers  indicate  test  specimens  that  are  not  glued  up. 


Nn 

Number  of 

Average  moisture  content  at  time 

test  pieces. 

of  test. 

1-20 
21-40 

20 
20 

Third  conditioning  room. 
Second  conditioning  room. 

41-60 

20 

First  conditioning  room. 

INTERNAL   STRESSES   IN   LAMINATED   CONSTRUCTION. 
GROUP  J. 


43 


All  laminations  to  contain  7  per  cent  moisture  at  the  time  of  gluing  and  be  of  a  compara- 
tively low  density.  Odd  numbers  indicate  test  specimens  that  are  glued  up;  even  numbers 
indicate  test  specimens  that  are  not  glued  up. 


No. 

Number  of 
test  pieces. 

Average  moisture  content  at  time 
of  test. 

1-  20 

20 

Glue  room  condition. 

21-  40 

20 

Work  shop  condition. 

41-  60 

20 

First  conditioning  room. 

61-  80 

20 

Second  conditioning  room. 

81-100 

20 

Third  conditioning  room. 

GROUP  K. 


All  laminations  to  contain  10  per  cent  moisture  at  the  time  of  gluing  and  be  of  compara- 
tively low  density.  Odd  numbers  indicate  test  specimens  that  are  glued  up;  even  numbers 
indicate  test  specimens  that  are  not  glued  up. 


No. 

Number  of 
test  pieces. 

Average  moisture  content  at  time 
of  test. 

1-  20 

20 

Glue  room  condition. 

21-  40 

20 

Work  shop  condition. 

41-  60 

20 

First  conditioning  room. 

61-  80 

20 

Second  conditioning  room. 

81-100 

20 

Third  conditioning  room. 

GROUP  L. 


All  laminations  to  contain  14  per  cent  moisture  at  the  time  of  gluing  and  be  of  low 
density.  Odd  numbers  indicate  test  specimens  that  are  glued  up;  even  numbers  indicate 
test  specimens  that  are  not  glued  up. 


No. 

Number  of 
test  pieces. 

Average  moisture  content  at  time 
of  test. 

1-  20 

20 

Glue  room  condition. 

21-  40 

20 

Work  shop  condition. 

41-  60 

20 

First  conditioning  room. 

61-  80 

20 

Second  conditioning  room. 

81-100 

20 

Third  conditioning  room. 

GROUP  M. 


All  laminations  to  contain  18  per  cent  moisture  at  the  time  of  gluing  and  be  of  low 
density.  Odd  numbers  indicate  test  specimens  that  are  glued  up;  even  numbers  indicate 
test  specimens  that  are  not  glued  up. 


No. 

Number  of 
test  pieces. 

Average  moisture  content  at  time 
of  test. 

1-  20 

20 

Glue  room  condition. 

21-  40 

20 

Work  shop  condition. 

41-  60 

20 

First  conditioning  room. 

61-  80 

20 

Second  conditioning  room. 

81-100 

20 

Third  conditioning  room. 

44 


REPORT   NATIONAL  ADVISORY   COMMITTEE   FOR   AERONAUTICS. 


GROUP  N. 


All  laminations  to  contain  7  per  cent  moisture  at  the  time  of  gluing  and  be  of  compara- 
tively low  density.  Odd  numbers  indicate  test  specimens  that  are  glued  up;  even  numbers 
indicate  test  specimens  that  are  not  glued  up. 


NO. 

Number  of 
test  pieces. 

Average  moisture  content  at  time 
of  test. 

1-20 
21-40 
41-60 

20 
20 

20 

Third  conditioning  room. 
Second  conditioning  room. 
First  conditioning  room. 

GROUP  O. 


All  laminations  to  contain  10  per  cent  moisture  at  the  time  of  gluing  and  be  of  comparatively 
low  density.  Odd  numbers  indicate  test  specimens  that  are  glued  up;  even  numbers  indicate 
test  specimens  that  are  not  glued  up. 


No 

Number  of 

Average  moisture  content  at  time 

test  pieces. 

of  test. 

1-20 

20 

Third  conditioning  room. 

21-40 

20 

Second  conditioning  room. 

41-60 

20 

First  conditioning  room. 

GROUP  P. 


All  laminations  to  contain  14  per  cent  moisture  at  the  time  of  gluing  and  be  of  a  compara- 
tively low  density.  Odd  numbers  indicate  test  specimens  that  are  glued  up;  even  numbers 
indicate  test  specimens  that  are  not  glued  up. 


"Mn 

Number  of 

Average  moisture  content  at  time 

test  pieces. 

of  test. 

1-20 

20 

Third  conditioning  room. 

21-40 

20 

Second  conditioning  room. 

41-60 

20 

First  conditioning  room. 

GROUP  R. 


All  laminations  to  contain  18  per  cent  moisture  at  the  time  of  gluing  and  be  of  compara- 
tively low  density.  Odd  numbers  indicate  test  specimens  that  are  glued  up;  even  numbers 
indicate  test  specimens  that  are  not  glued  up. 


No. 

Number  of 
test  pieces. 

Average  moisture  content  at  time 
of  test. 

1-20 
21-40 
41-60 

20 
20 
20 

Third  conditioning  room. 
Second  conditioning  room. 
First  conditioning  room. 

GROUP  S. 

All  laminations  to  contain  7  per  cent  moisture  at  the  time  of  gluing  and  be  of  a  mixed 
density.  Odd  numbers  indicate  test  specimens  that  are  glued  up;  even  numbers  test  speci- 
mens that  are  not  glued  up. 


No. 

Number  of 
test  pieces. 

Average  moisture  content  at  time 
of  test. 

1-  20 

20 

Glue  room  conditions. 

21-  40 

20 

Workshop  conditions. 

41-  60 

20 

First  conditioning  room. 

61-  80 

20 

Second  conditioning  room  . 

81-100 

20 

Third  conditioning  room  . 

INTERNAL   STRESSES   IN    LAMINATED   CONSTRUCTION. 
GROUP  T. 


45 


All  laminations  to  contain  10  per  cent  moisture  at  the  time  of  gluing  and  be  of  a  mixed 
density.  Odd  numbers  indicate  test  specimens  that  are  glued  up;  even  numbers  indicate  test 
specimens  that  are  not  glued  up. 


No. 

Number  of 
test  pieces. 

Average  moisture  content  at  time 
of  test. 

1-  20 

20 

Glue  room  conditions. 

21-  40 

20 

Workshop  conditions. 

41-  60 

20 

First  conditioning  room. 

61-  80 

20 

Second  conditioning  room. 

81-100 

20 

Third  conditioning  room. 

GROUP  U. 


All  laminations  to  contain  14  per  cent  moisture  at  the  time  of  gluing  and  be  of  a  mixed 
density.  Odd  numbers  indicate  test  specimens  that  are  glued  up;  even  numbers  indicate  test 
specimens  that  are  not  glued  up. 


No. 

Number  of 
test  pieces. 

Average  moisture  content  at  time 
of  test. 

i-  20 

20 

Glue  room  conditions. 

21-  40 

20 

Workshop  conditions. 

41-  60 
61-  80 

20 

20 

First  conditioning  room. 
Second  conditioning  room. 

81-100 

20 

Third  conditioning  room. 

GROUP  V. 


All  laminations  to  contain  18  per  cent  moisture  at  the  time  of  gluing  and  be  of  a  mixed 
density.  Odd  numbers  indicate  test  specimens  that  are  glued  up;  even  numbers  indicate  test 
specimens  that  are  not  glued  up. 


No. 

Number  of 
test  pieces. 

Average  moisture  content  at  time 
of  test. 

1-  20 

20 

Glue  room  conditions. 

21-  40 

20 

Workshop  conditions. 

41-  60 

20 

First  conditioning  room. 

61-  80 

20 

Second  conditioning  room  . 

81-100 

20 

Third  conditioning  room. 

GROUP  W. 


All  laminations  to  contain  7  per  cent  moisture  at  the  time  of  gluing  and  be  of  a  mixed 
density.  Odd  numbers  indicate  test  specimens  that  are  glued  up;  even  numbers  indicate 
test  specimens  that  are  not  glued  up. 


Number  of 

Average  moisture  content  at  time 

test  pieces.  • 

of  test. 

1-20 

20 

Third  conditioning  room. 

21-40 

20 

Second  conditioning  room. 

41-60 

20 

First  conditioning  room. 

46 


REPORT   NATIONAL  ADVISORY   COMMITTEE   FOR  AERONAUTICS. 

GROUP  X. 


All  laminations  to  contain  10  per  cent  moisture  at  the  time  of  gluing  and  be  of  a  mixed 
density.  Odd  numbers  indicate  test  specimens  that  are  glued  up;  even  numbers  indicate 
test  specimens  that  are  not  glued  up. 


No. 

Number  of 
test  pieces. 

Average  moisture  content  at  time 
of  test. 

1-20 
21-40 
41-60 

20 
20 
20 

Third  conditioning  room. 
Second  conditioning  room. 
First  conditioning  room. 

GROUP  Y. 

All  laminations  to  contain  14  per  cent  moisture  at  the  time  of  gluing  and  be  of  a  mixed 
density.  Odd  numbers  indicate  test  specimens  that  are  glued  up;  even  numbers  indicate 
test  specimens  that  are  not  glued  up. 


No. 

Number  of 
test  pieces. 

Average  moisture  content  at  time 
of  test. 

1-20 
21-40 
41-60 

20 
20 
20- 

Third  conditioning  room. 
Second  conditioning  room. 
First  conditioning  room. 

GROUP  Z. 

All  laminations  to  contain  18  per  cent  moisture  at  the  time  of  gluing  and  be  of  a  mixed 
density.  Odd  numbers  indicate  test  specimens  that  are  glued  up;  even  numbers  indicate  test 
specimens  that  are  not  glued  up. 


"Mr* 

Number  of 

Average  moisture  content  at  time 

test  pieces. 

of  test. 

1-20 

20 

Third  conditioning  room. 

21-40 

20 

Second  conditioning  room. 

41-60 

20 

First  conditioning  room. 

SERIES  3.— VARIATION  IN  MOISTURE  CONTENT. 

BIRCH,  OAK,  AND  MAHOGANY  ARE  TO  BE  TESTED. 

Preparation  of  test  pieces. — The  laminations  of  any  one  test  piece  are  to  be  all  plain  sawed 
or  all  quarter  sawed  and  of  the  same  density  and  rate  of  growth.  The  variations  in  moisture 
content  are  to  be  obtained  either  by  drying  (under  conditions  slightly  more  severe  than  air 
drying)  in  the  laboratory  or  by  placing  the  specimen  in  one  of  the  conditioning  rooms,  pro- 
viding temperature  and  moisture  conditions  as  required.  Each  test  specimen  will  be  con- 
ditioned in  consecutive  rooms,  passing  through  all  of  the  conditions  preceding  that  condition 
at  which  the  test  piece  is  to  be  broken. 

GROUP  A. 

Average  moisture  content  throughout  the  test  pieces  at  the  time  of  testing  to  be  that  pro- 
duced by  conditioning  in  the  glue  room.  Odd  numbers  indicate  test  pieces  that  are  glued  up; 
even  numbers  indicate  test  pieces  that  are  not  glued  up. 

Conditions  at  the  time  the  test  puces  are  glued  together. 


Moisture  per  cent  when  glued. 

No. 

Number  of 
pieces. 

Center. 

Sides. 

1-20 

20 

7 

7 

21-40' 

20 

7 

10 

41-60 

20 

7 

14 

61-80 

20 

7 

18 

INTERNAL   STRESSES   IN   LAMINATED   CONSTRUCTION. 
GROUP  B. 


47 


Average  moisture  content  throughout  the  test  pieces  at  the  time  of  testing  to  be  that  pro- 
duced by  conditioning  in  the  workshop.  Odd  numbers  indicate  test  pieces  that  are  glued  up; 
even  numbers  indicate  test  pieces  that  are  not  glued  up. 

Conditions  at  the  time  the  test  pieces  are  glued  together. 


Moisture  per  cent  when  glued. 

No. 

Number  of 
pieces. 

i 

Center. 

Sides. 

1-20 

20 

7 

7 

21-40 

20 

7 

10 

41-60 

20 

7 

14 

61-80 

20 

7 

18 

GROUP  C. 


Average  moisture  content  throughout  the  test  pieces  at  the  time  of  testing  to  be  that  pro- 
duced by  conditioning  in  the  first  conditioning  room  after  conditioning  in  the  workshop.  Odd 
numbers  indicate  test  specimens  that  are  glued  up;  even  numbers  indicate  test  specimens 

that  are  not  glued  up. 

Conditions  at  the  time  the  test  pieces  are  glued  together. 


Moisture  per  cent  when  glued. 

Number  of 

pieces. 

Center. 

Sides. 

1-20 

20 

7 

7 

21-40 

20 

7 

10 

41-60 

20 

7 

14 

61-80 

~26 

7 

'18 

GROUP  D. 

Average  moisture  content  throughout  the  test  pieces  at  the  time  of  testing  to  be  that  pro- 
duced by  conditioning  in  second  conditioning  room  after  conditioning  in  the  workshop  and  the 
required  period  of  tune  in  the  first  conditioning  room.  Odd  numbers  indicate  test  specimens 
that  are  glued  up;  even  numbers  indicate  test  specimens  that  are  not  glued  up. 

Conditions  at  the  tine  the  test  pieces  are  glued  together. 


Moisture  per  cent  when  glued. 

No. 

Number  of 
pieces. 

. 

Center. 

Sides. 

1-20 

20 

7 

7 

21-40 

20 

7 

10 

41-60 

20 

7 

14 

61-80 

20 

7 

18 

GROUP  E. 

Average  moisture  content  throughout  the  test  pieces  at  the  time  of  testing  to  be  that  pro- 
duced by  conditioning  in  third  conditioning  room  after  conditioning  in  the  workshop  and  the 
required  period  of  time  in  the  first  and  second  conditioning  rooms.  Odd  numbers  indicate 
test  specimens  that  are  glued  up;  even  numbers  indicate  test  specimens  that  are  not  glued  up. 


48 


EEPOET   NATIONAL,  ADVISORY   COMMITTEE   FOR   AERONAUTICS. 
Conditions  at  the  time  the  test  pieces  are  glued  together. 


,  Moisture  per  cent  when  glued. 

No. 

Number  of 
pieces. 

Center. 

fides. 

1-20 

20 

7 

7 

21-40 

20 

7 

10 

41-60 

20 

7 

14 

61-80 

20 

7 

18 

GROUP  F. 


Average  moisture  content  throughout  the  test  pieces  at  the  time  of  testing  to  be  that  pro- 
duced by  conditioning  in  the  third  conditioning  room  after  conditioning  in  the  workshop.  Odd 
numbers  indicate  test  specimens  that  are  glued  up;  even  numbers  indicate  test  specimens  that 

are  not  glued  up. 

Conditions  at  the  time  the  test  pieces  are  glued  together. 


Moisture  per  cent  when  glued. 

Number  of 

pieces. 

Center. 

Sides. 

1-20 

20 

7 

7 

21-40 

20 

7 

10 

41-60 

20 

7 

14 

61-80 

20 

7 

18 

GROUP  G. 

Average  moisture  content  throughout  the  test  pieces  at  the  time  of  testing  to  be  that 
produced  by  conditioning  in  the  second  conditioning  room  after  conditioning  in  the  workshop 
and  the  required  period  of  time  in  the  third  conditioning  room.  Odd  numbers  indicate  test 
specimens  that  are  glued  up;  even  numbers  indicate  test  specimens  that  are  not  glued  up. 

Conditions  at  the  time  the  test  pieces  are  glued  together. 


Moisture  per  cent  when  glued. 

Number  of 

pieces. 

Center. 

Sides. 

1-20 

20 

7 

7 

21-40 

20 

7 

10 

41-60 

20 

7 

14 

61-80 

20 

7 

18 

GROUP  H. 

Average  moisture  content  throughout  the  test  pieces  at  the  time  of  testing  to  be  that 
produced  by  conditioning  in  the  first  conditioning  room  after  conditioning  in  the  workshop 
and  the  required  period  of  time  in  the  third  and  second  conditioning  rooms.  Odd  numbers 
indicate  test  specimens  that  are  glued  up;  even  numbers  indicate  test  specimens  that  are 
not  glued  up. 

Conditions  at  the  time  the  test  pieces  are  glued  together. 


Moisture  per  cent  when  glued. 

No. 

Number  of 
pieces. 

Center. 

Fides. 

1-20 

20 

7 

7 

21-40 

20 

7 

10 

41-60 

20 

7 

14 

61-80 

20 

7 

18 

INTERNAL   STRESSES   IN   LAMINATED   CONSTRUCTION. 
GROUP  J. 


49 


Average  moisture  content  throughout  the  test  pieces  at  the  time  of  testing  to  be  that 
produced  by  conditioning  in  the  glue  room.  Odd  numbers  indicate  test  specimens  that  are 
glued  up;  even  numbers  indicate  test  specimens  that  are  not  glued  up. 

Conditions  at  the  time  the  test  pieces  are  glued  together. 


Moisture  per  cent  when  glued. 

Number  of 

pieces. 

Center. 

Sides. 

1-20 

20 

10 

7 

21-10 

20 

10 

10 

41-60 

20 

10 

14 

61-80 

20                     10 

18 

GROUP  K. 


Average  moisture  content  throughout  the  test  pieces  at  the  time  of  testing  to  be  that 
produced  by  conditioning  in  the  workshop.  Odd  numbers  indicate  test  specimens  that  are 
glued  up;  even  numbers  indicate  test  specimens  that  are  not  glued  up. 

Conditions  at  the  time  the  test  pieces  are  glued  together. 


Moisture  per  cent  when  glued. 

Number  of 

pieces. 

Center. 

Sides. 

1-20 

20 

10 

7 

21-40 

20- 

10 

10 

41-60 

20 

10 

14 

61-80 

20 

10 

18 

1 

GROUP  L. 

Average  moisture  content  throughout  the  test  pieces  at  the  time  of  testing  to  be -that 
produced  by  conditioning  in  first  conditioning  room  after  conditioning  in  the  workshop.  Odd 
numbers  indicate  test  specimens  that  are  glued  up;  even  numbers  indicate  test  specimens 

that  are  not  glued  up. 

Conditions  at  the  time  the  test  pieces  are  glued  together. 


Moisture  per  cent  when  glued. 

No. 

Number  of 
pieces. 

Center. 

Sides. 

1-20 

20 

10 

7 

21-40 

20 

10 

10 

41-60 

20 

10 

14 

61-80 

20 

10 

18 

GROUP  M. 


Average  moisture  content  throughout  the  test  pieces  at  the  time  of  testing  to  be  that 
produced  by  conditioning  in  second  conditioning  room  after  conditioning  in  the  workshop  and 
the  required  period  of  time  in  the  first  conditioning  room.  Odd  numbers  indicate  test  speci- 
mens that  are  glued  up;  even  numbers  indicate  test  specimens  that  are  not  glued  up. 


50 


REPORT   NATIONAL  ADVISORY   COMMITTEE  FOR  AERONAUTICS. 
Conditions  at  the  time  the  test  pieces  are  glued  together. 


Moisture  per  cent  when  glued. 

No. 

Number  of 
pieces. 

Center. 

Sides. 

1-20 

20 

10 

7 

21-40 

20 

10 

10 

41-60 

20 

10 

14 

61-80 

20 

10 

18 

GROUP  N. 

Average  moisture  content  throughout  the  test  pieces  at  the  time  of  testing  to  be  that  pro- 
duced by  conditioning  in  third  conditioning  room  after  conditioning  in  the  workshop  and  the 
required  periods  of  time  in  the  first  and  second  conditioning  rooms.  Odd  numbers  indicate  test 
specimens  that  are  glued  up;  even  numbers  indicate  test  specimens  that  are  not  glued  up. 

Conditions  at  the  time  the  test  pieces  are  glued  together. 


Moisture  per  cent  when  glued. 

Number  of 

pieces. 

Center. 

Sides. 

1-20 

20 

10 

7 

21-40 

20 

10 

10 

41-60 

20 

10 

14 

61-80 

20 

10 

18 

GROUP  P. 

Average  moisture  content  throughout  the  test  pieces  at  the  time  of  testing  to  be  that  pro- 
duced by  final  conditioning  in  the  third  conditioning  room  after  conditioning  in  the  workshop. 
Odd  numbers  indicate  test  specimens  that  are  glued  up;  even  numbers  indicate  test  specimens 
that  are  not  glued  up. 

,        Conditions  at  the  time  the  test  pieces  are  glued  together. 


Moisture  per  cent  when  glued. 

Number  of 

pieces. 

Center. 

Sides. 

1-20 

20 

10 

7 

21-40 

20 

10 

10 

41-60 

20 

10 

14 

61-80 

20 

10 

18 

GROUP  R. 

Average  moisture  content  throughout  the  test  pieces  at  the  time  of  testing  to  be  that  pro- 
duced by  final  conditioning  in  the  second  conditioning  room  after  conditioning  in  the  workshop 
and  the  required  period  of  time  in  the  third  conditioning  room.  Odd  numbers  indicate  test 
specimens  that  are  glued  up;  even  numbers  indicate  test  specimens  that  are  not  glued  up. 

Conditions  at  the  time  the  test  pieces  are  glued  together. 


Moisture  per  cent  when  glued. 

Number  of 

pieces. 

Center. 

Sides. 

1-20 

20 

10 

7 

21-40. 

20 

10 

10 

41-60 

20 

10 

14 

61-80 

20 

10 

18 

INTERNAL   STRESSES   IN   LAMINATED   CONSTRUCTION. 


51 


GROUP  S. 

Average  moisture  content  throughout  the  test  pieces  at  the  time  of  testing  to  be  that 
produced  by  final  conditioning  in  the  first  conditioning  room  after  conditioning  in  the  workshop 
and  the  required  period  of  time  in  the  third  and  second  conditioning  rooms.  Odd  numbers 
indicate  test  specimens  that  are  glued  up;  even  numbers  indicate  test  specimens  that  are  not 

glued  up. 

Conditions  at  the  time  the  test  pieces  are  glued  together. 


Moisture  per  cent  when  glued. 

Number  of 

pieces. 

Center. 

Sides. 

1-20 

20 

10 

7 

21-40 

20 

10 

10 

41-60 

20 

10 

14 

61-80 

20 

10 

18 

GROUP  T. 


Average  moisture  content  throughout  the  test  pieces  at  the  time  of  testing  to  be  that 
produced  by  conditioning  in  the  glue  room.  Odd  numbers  indicate  test  specimens  that  are  glued 
up:  even  numbers  indicate  test  specimens  that  are  not  glued  up. 

Conditions  at  the  time  the  test  pieces  are  glued  together. 


Moisture  per  cent  when  glued. 

Number  of 

pieces. 

Center. 

fides. 

1-20 

20 

14 

7- 

21-40 

20 

14 

10 

41-60 

--      20 

14 

14 

61-80 

20 

14 

18 

GROUP  U. 


Average  moisture  content  throughout  the  test  pieces  at  the  time  of  testing  to  be  that  pro- 
duced by  final  conditioning  in  the  workshop.  Odd  numbers  indicate  test  specimens  that  are 
glued  up;  even  numbers  indicate  test  specimens  that  are  not  glued  up. 

Conditions  at  the  time  the  test  pieces  are  glued  together. 


Moisture  per  cent  when  glued. 

Number  of 

pieces. 

Center. 

Sides. 

1-20 

20 

14 

7 

21-40 

20 

14 

10 

41-60 

20 

14 

14 

61-80 

20 

14 

18 

GROUP  W. 


Average  moisture  content  throughout  the  test  pieces  at  the  time  of  testing  to  be  that 
produced  by  final  conditioning  in  the  first  conditioning  room  after  conditioning  in  the  workshop. 
Odd  numbers  indicate  test  specimens  that  are  glued  up;  even  numbers  indicate  test  specimens 
that  are  not  glued  up. 


52 


REPORT   NATIONAL  ADVISORY   COMMITTEE  FOR  AERONAUTICS. 
Conditions  at  the  time  the  test  pieces  are  glued  together. 


Moisture  per  cent  when  glued. 

Number  of 

pieces. 

Center. 

Sides. 

1-20 

20 

14 

7 

21-40 

20 

14 

10 

41-60 

20 

14 

14 

61-80 

20 

14 

18 

GROUP  X. 

Average  moisture  throughout  the  test  pieces  at  the  time  of  testing  to  be  that  produced 
by  final  conditioning  in  the  second  conditioning  room  after  conditioning  in  the  workshop  and 
the  required  period  of  time  in  the  first  conditioning  room.  Odd  numbers  indicate  test  specimens 
that  are  glued  up;  even  numbers  indicate  test  specimens  that  are  not  glued  up. 

Conditions  at  the  time  the  test  pieces  are  glued  together. 


Moisture  per  cent  when  glued. 

Number  of 

pieces. 

Center. 

Sides. 

1-20 

20 

14 

7 

21-40 

20 

14 

10 

41-60 

20 

14 

14 

61-80 

20 

14 

18 

GROUP  Y. 

Average  moisture  content  throughout  the  test  pieces  at  the  time  of  testing  to  be  that  pro- 
duced by  final  conditioning  in  the  third  conditioning  room  after  conditioning  in  the  workshop 
and  the  required  period  of  time  in  the  first  and  second  conditioning  rooms.  Odd  numbers 
indicate  test  specimens  that  are  glued  up;  even  numbers  indicate  test  specimens  that  are  not 

glued  up. 

Conditions  at  the  time  the  test  pieces  are  glued  together. 


Moisture  per  cent  when  glued. 

Number  of 

pieces  . 

Center. 

Sides. 

1-20 

20 

14 

7 

21-40 

20 

14 

10 

41-60 

20 

14 

14 

61-80 

20 

14 

18 

GROUP  Z. 

Average  moisture  content  throughout  the  test  pieces  at  the  time  of  testing  to  be  that  pro- 
duced by  final  conditioning  in  third  conditioning  room  after  conditioning  in  the  workshop. 
Odd  numbers  indicate  test  specimens  that  are  glued  up;  even  numbers  indicate  test  specimens 

that  are  not  glued  up. 

Conditions  at  the  time  the  test  pieces  are  glued  together. 


No. 


1-20 
21-40 
41-60 
61-80 


Number  of 

Moisture  per  cent  when  glued. 

pieces. 

Center. 

Sides. 

20 

14 

7 

20 

14 

10 

20 

14 

14 

20 

14 

18 

INTERNAL   STRESSES   IN   LAMINATED   CONSTRUCTION. 
GROUP  AA. 


53 


Average  moisture  content  throughout  the  test  pieces  at  the  time  of  testing  to  be  that 
produced  by  final  conditioning  in  second  conditioning  room  after  conditioning  in  the  workshop 
and  the  required  period  of  time  in  the  third  conditioning  room.  Odd  numbers  indicate  test 
specimens  that  are  glued  up;  even  numbers  indicate  test  specimens  that  are  not  glued  up. 

Conditions  at  the  time  the  test  pieces  are  glued  together. 


Moisture  per  cent  when  glued. 

Number  of 

pieces. 

Center. 

Sides. 

1-20 

20 

14 

7 

21-40 

20 

14 

10 

41-60 

20 

14 

14 

61-80 

20 

14 

18 

GROUP  BB. 

Average  moisture  content  throughout  the  test  pieces  at  the  time  of  testing  to  be  that  pro- 
duced by  final  conditioning  in  first  conditioning  room  after  conditioning  in  the  workshop  and  the 
required  periods  of  time  in  the  third  and  second  conditioning  rooms.  Odd  numbers  indicate 
test  specimens  that  are  glued  up;  even  numbers  indicate  test  specimens  that  are  not  glued  up. 

Conditions  at  the  time  the  test  pieces  are  glued  together. 


Moisture  per  cent  when  glued. 

Number  of 

pieces. 

Center. 

Sides. 

1-20 

20 

14 

7 

21-40 

-      20 

14 

10 

41-60 

20 

14 

14 

61-80 

20      . 

14 

18 

GROUP  CO. 


Average  moisture  content  throughout  the  test  pieces  at  the  time  of  testing  to  be  that  pro- 
duced by  final  conditioning  in  the  glue  room.  Odd  numbers  indicate  test  specimens  that  are 
glued  up;  even  numbers  indicate  test  specimens  that  are  not  glued  up. 

Conditions  at  the  time  the  test  pieces  are  glued  together. 


Moisture  per  cent  when  glued. 

Number  of 

pieces. 

Center. 

Sides. 

1-20 

20 

18 

7 

21-40 

20 

18 

10 

41-60 

20 

18 

14 

61-80 

20 

18 

18 

GROUP  DD. 


Average  moisture  content  throughout  the  test  pieces  at  the  time  of  testing  to  be  that  pro- 
duced by  final  conditioning  in  the  workshop.  Odd  numbers  indicate  test  specimens  that  are 
glued  up;  even  numbers  indicate  test  specimens  that  are  not  glued  up. 


54 


REPORT   NATIONAL  ADVISORY   COMMITTEE   FOR  AERONAUTICS. 
Conditions  at  the  time  the  test  pieces  are  glued  together. 


Moisture  per  cent  when  glued. 

M/\ 

Number  of 

pieces 

Center. 

Sides. 

1-20 

20 

18 

7 

21^0 

20 

18 

10 

41-60 

20 

18 

14 

61-80 

20 

18 

18 

GROUP  EE. 


Average  moisture  content  throughout  the  test  pieces  at  the  time  of  testing  to  be  that  pro- 
duced by  final  conditioning  in  the  first  conditioning  room  after  conditioning  in  the  workshop. 
Odd  numbers  indicate  test  specimens  that  are  glued  up;  even  numbers  indicate  test  specimens 

that  are  not  glued  up. 

Conditions  at  the  time  the  test  pieces  are  glued  together. 


Moisture  per  cent  when  glued. 

Number  of 

pieces. 

Center. 

fides. 

1-20 

20 

18 

7 

21-40 

20 

18 

10 

41-60 

20 

18 

14 

61-80 

20 

18 

18 

GROUP  FF. 

Average  moisture  content  throughout  the  test  pieces  at  the  time  of  testing  to  be  that  pro- 
duced by  final  conditioning  in  the  second  conditioning  room  after  conditioning  in  the  workshop 
and  the  required  period  of  time  in  the  first  conditioning  room.  Odd  numbers  indicate  test 
specimens  that  are  glued  up;  even  numbers  indicate  test  specimens  that  are  not  glued  up. 

Conditions  at  the  time  the  test  pieces  are  glued  together. 


Moisture  per  cent  when  glued. 

Number  of 

pieces. 

Center. 

Sides 

1-20 

20 

18 

7 

21-40 

20 

18 

10 

41-60 

20 

18 

14 

61-80 

20 

18 

18 

GROUP  GG. 

Average  moisture  content  throughout  the  test  pieces  at  the  time  of  testing  to  be  that  pro- 
duced by  final  conditioning  in  the  third  conditioning  room  after  conditioning  in  the  workshop 
and  the  required  period  of  time  in  the  first  and  second  conditioning  rooms.  Odd  numbers  indicate 
test  specimens  that  are  glued  up;  even  numbers  indicate  test  specimens  that  are  not  glued  up. 

Conditions  at  the  time  the  test  pieces  are  glued  together. 


Moisture  per  cent  when  glued. 

XTrt 

Number  of 

JNO. 

pieces. 

Center. 

Sides. 

1-20 

20 

18 

7 

21-40 

20 

18 

10 

41-60 

20 

18 

14 

61-80 

20 

18 

18 

INTERNAL   STRESSES   IN   LAMINATED   CONSTRUCTION. 
GROUP   HH. 


55 


Average  moisture  content  throughout  the  test  pieces  at  the  time  of  testing  to  be  that  pro- 
duced by  final  conditioning  in  third  conditioning  room  after  conditioning  in  the  workshop. 
Odd  numbers  indicate  test  specimens  that  are  glued  up;  even  numbers  indicate  test  specimens 

that  are  not  glued  up. 

Conditions  at  the  time  the  test  pieces  are  glued  together. 


Moisture  per  cent  when  glued. 

No. 

Number  of 
pieces. 

Center. 

Sides. 

1-20 

20 

18 

7 

21-40 

20 

18 

10 

41-60 

20 

18 

14 

61-80 

20 

18 

18 

GROUP  KK. 

Average  moisture  content  throughout  the  test  pieces  at  the  time  of  testing  to  be  that  pro- 
duced by  final  conditioning  in  second  conditioning  room  after  conditioning  in  the  workshop 
and  the  required  period  of  time  in  third  conditioning  room.  Odd  numbers  indicate  test  speci- 
mens that  are  glued  up;  even  numbers  indicate  test  specimens  that  are  not  glued  up. 

Conditions  at  the  time  the  test  pieces  are  glued  together. 


Moisture  per  cent  when  glued. 

Number  of 

pieces. 

Center. 

Sides. 

1-20 

20 

18 

7 

21-40 

20 

18 

10 

41-60 

20 

18 

14 

61-80 

20 

18 

18 

GROUP  MM. 

Average  moisture  content  throughout  the  test  pieces  at  the  time  of  testing  to  be  that  pro- 
duced by  final  conditioning  in  first  conditioning  room  after  conditioning  in  the  workshop  and 
the  required  periods  of  time  in  the  third  and  second  conditioning  rooms.  Odd  numbers  indicate 
test  specimens  that  are  glued  up;  even  numbers  indicate  test  specimens  that  are  not  glued  up. 

Conditions  at  the  time  the  test  pieces  are  glued  together. 


- 

Moisture  per  cent  when  glued. 

Number  of 

pieces. 

Center. 

Sides. 

1-20 

20 

18 

7 

21-40 

20 

18 

10 

41-60 

20 

18 

14 

61-80 

.  2,0 

18 

18 

56 


REPORT   NATIONAL  ADVISORY   COMMITTEE   FOR  AERONAUTICS. 


APPENDIX  B.— SAMPLES  OF  RECORD  FORMS. 

SERIES  1.— MATCHING  PLAIN  AND  QUARTER  SAWED. 

Specimens  are  to  be  conditioned  or  brought  to  constant  weight  in  each  of  the  various 
rooms  in  the  order  given  below,  then  taken  out  and  tested,  e.  g.,  Group  E  will  be  conditioned 
in  the  glue  room,  workshop,  first,  second,  and  third  conditioning  rooms  and  then  tested. 

Group. 

Glue  room A. 

Workshop B. 

First  conditioning  room C. 

Second  conditioning  room D. 

Third  conditioning  room E. 

Glue  room. 
Workshop. 

Third  conditioning  room F. 

Second  conditioning  room G. 

First  conditioning  room H. 

SERIES  2.— DENSITY  DIFFERENCE. 

Specimens  are  to  be  conditioned  or  brought  to  constant  weight  in  each  of  the  various 
rooms  in  the  order  given  below  until  they  reach  constant  weight  in  the  room  opposite  which 
their  numbers  appear.  They  will  then  be  tested. 


Groups  A,  B,  C,  D. 

Groups  J,  K,  L,  M. 

Groups  S,  T,  U,  V. 

Specimen 
numbers. 

Glue  room  

1-  20 
21-  40 
41-  60 
61-  80 
81-100 

Workshop  

First  conditioning 
Second  condition! 
Third  conditionin 

room  

ng  room  

groom  

Groups  E,  F,  G,  H.i 

Groups  N,  O,  P,  R.» 

Groups  W,  X,  Y,  Z.J 

Specimen 
numbers. 

Glue  room  

Workshop  

Third  conditionin 
Second  conditioni 
First  conditioning 

s  room  .  .  . 

1-20 
21-40 
41-60 

ng  room  

room  

1  Comparatively  high  density. 


3  Comparatively  low  density. 


Mixed  density. 


SERIES  3.— VARIATION  IN  MOISTURE  CONTENT. 

Specimens  are  to  be  conditioned  or  brought  to  constant  weight  in  each  of  the  various 
rooms  in  the  order  given  below,  then  taken  out  and  tested,  e.  g.,  Groups  E,  N,  Y,  and  GG  will 
be  conditioned  in  the  glue  room,  workshop,  first,  second,  and  third  conditioning  rooms  and 
then  tested. 

Groups. 

Glue  room ...A,  J,  T,  CO. 

Workshop .B,  K,  U,  DD. 

First  conditioning  room C,  L,  W.  EE. 

Second  conditioning  room D,  M,  X,  FF. 

Third  conditioning  room E,  N,  Y,  GG. 

Glue  room. 
Workshop. 

Third  conditioning  room F.  P.  Z,  HH. 

Second  conditioning  room G ,  R,  AA,  KK.         . 

First  conditioning  room H,  S,  BB,  MM. 


ADDITIONAL  COPIES 

OF  THIS  PUBLICATION  MAY  BE  PROCURED  FROM 

THE  SUPERINTENDENT  OF  DOCUMENTS 

GOVERNMENT  PRINTING  OFFICE 

WASHINGTON,  D.  C. 

AT 

Oft  CENTS  PER  COPY 


6 


LL>  27 

I~- 


5190 


UNIVERSITY  OF  CALIFORNIA  LIBRARY 


